As with most hero stories, the tales of his bravery were greatly exaggerated over time. He later admits that he himself had started the exaggeration. While his original stories had him luckily defeating the monsters, he actually stumbled across them already defeated, and just ensured they were gone before extracting a talisman.
As a ruler, he was summoned to fight a pillager that was called dragon. This would be actual combat, so he could not rely on luck. However, he could use the talismen that he procured and would defeat him in a David vs. Goliath contest. This probably contributed to his reputation as a "dragon slayer". It was only later (in a time not covered by the book) that he would actually fight dragons.
The takeaway seems to be that nothing ventured, nothing gained. There were plenty of other legitimate warriors. They could have gone out and gathered the rewards he did. However, none of them were willing to go. Being willing lead him to the initial rewards. However, that was not enough. He would have failed the final battle if he had not used the experience from what he previously accomplished.
The conclusion to Dragonwatch brings everything to a conclusion and ties up many of the loose ends. The characters complete many great journeys. While Kendra's journey was month important to Fablehaven, Seth's journey is key to Dragonwatch. He undergoes the great quest and completes the hero's journey. The journey includes many fantasy and religious themes with a twist. Seth must journey to the darkest of dark realms and suffer immensely before he can finally be submerged in the water that heals. An innocent being would serve as the guide to bring him there. An immortal being must willingly end his life to help restore that which was broken. 
The Titan Games are a popular event lead by Humbuggle, a powerful troll. A giant stadium brings people from all over. There they watch gladiators battle. It is all part of of a long scheme of games to allow the victor to take a precious stone which grants immense power. Seth goes there in an attempt to gain his memories back. In the process he runs into separated memories (that he helps cristen as "Reggie"), a satyr professor and many other beings. 
Seth joins the darkside in this third Dragonwatch book. A "dark unicorn" took him away and has him continue to run errands to support the undead and other dark creatures. Seth has lost his memories and is not sure who is telling the truth. He feels it is bad to have people imprisoned, so helps free the creatures and run other errands for the dark.
The dragons have declared war. They want full control and don't want to be controlled by humans. They are after a special stone that will all them this power. The humans don't want them to have it. They are seeking it also. However, the troll guarding it has other things in mind. Those that do something bad on their own are forever cursed by his game, but those that had something bad happen out of their control are restored. We also get to see many more fantastical creatures, such as talking animals and a living forest. The cousins gain the ability to see the magical creatures, but cause some havoc on their own. A "memory loss" door causes interesting things to happen. Like a typical middle book, it opens more questions than it closes.
The Dragonwatch series kicks off where Fablehaven ended. Seth goes on a mission to return the sword, and discovers that he will have a future with Dragons. Back at Fablehaven, the kids are with both sets of grandparents. Things get interesting when their cousins visit. They don't believe in the fairy stuff. However, this eventually becomes a benefit when hunting for gold or playing a "super VR capture the flag".
The final Fablehaven book ends with a giant battle between the "good guys" and the demons. Leading up to it, there is plenty of trickery that leads to people accidentally helping out those on the other side. Adventures take the main characters all across the globe in a quest to prevent the opening of the demon prison. Alas, the effort is futile.
Book four of Fablehaven starts with Kendra and Seth living a fairly normal life as teenagers. (Though they do have some bodyguards in place.) However, things quickly start to change. Kendra gets pricked by a stingbulb and ends up with a cloned version of herself. (There is eventually another clone.) There is also a magic knapsack that has a large expanded space inside. 
Things are getting weird at Fablehaven. There seems to be a plague of darkness. People and magical creatures are turning dark. It will take some special magic to get things back to normal. Meanwhile, there is a quest to find other missing artifacts to protect a prison. Things do not go so well. However, they do bring back a person from the past to help in the quest to free Fablehaven from evil. They also get some more clarity on who is on the good side - but then get more confusion as well. There is plenty to look forward to in the next book.
Book two of Fablehaven opens up new adventures in the typical "second book" manner. The first book seemed to bring things to a conclusion. However, new adventures are now coming. The book starts with the children at school. It seems pretty normal until a hideous boy shows up in Kendra's class. Only it turns out that only she can see his ugliness. Everyone else sees him as super cute. But once they try to kiss him, they are repulsed by his breath. 
In Spirit Animals, Brandon Mull breaks from his typical "modern fantasy" approach and goes into pure fantasy. The world is somewhat similar to ours, with kingdoms that resemble China and other places, but it is clearly its own thing. People have the opportunity to summon "Spirit Animals" when they come of age. These are animals that have a connection to great people of the past. At the time of the story, there is a great calamity unfolding, so more spirit animals are summoned. The "Green Cloaks" are attempting to gather the great summoners together to help save the world. The kids that have summoned these animals are from a broad spectrum of society. One is an elite that are used to summoning animals. Another is an orphan who was surprised, yet saw it as a ticket out of trouble. They each have their doubts about what is going on, and even question whether they are on the side of the good guys or bad guys. Then the book ends, clearly waiting to be picked up in the next book of the series. Currently, this ranks as my least favorite of the Mull series. I prefer his books that attempt to connect to our modern world.
Death Weavers is the 4th book in the five Kingdom's series. Here, our team spends much of their time with "echos". These are "Spirits" of people after they have died, but before they have moved on to a final place. They can spend as much time as they want in the echo world. However, they do hear a call to go down the river to the exit of the world. Music is a powerful force in the echo world. Different beings have music that signals what they are doing. Different people also hear different music at different times. On special occasions, people can go into the world as "bright echos" who are still alive in the regular world.
Things continue in the Five Kingdom's series. Here they voyage into a kingdom of advanced technology. At one time a computer even managed to assume power. However, this computer was eventually subjugated. 
The Rogue Knight continues where the first book in the Five Kingdom series left off. The "gang" goes on the quest to find the master shaper and work to "save the world". In the process they run into more people that had come from Earth, including some that our protagonist knew, as well as others that had disappeared at separate times. However, other than mentioning these other "earthlings", there is no connection to the real world. 
Chasing the Prophecy is the conclusion of the Beyonders series. It is non-stop action, with the heroes finally completing their quest. There are plenty of twists and turns. However, some parts stretch credulity (Would somebody thousands of years ago really be able to see the name and baseball affinity of a boy from another world?) Other key points seem to occur suddenly without much buildup. (They seem to instantly realize how to destroy a "substance changing" monster.) 
Seeds of Rebellion is a typical "second book" in a series. The first book reached a conclusion, with the "hero" returning to his world at the end. It could stand alone (though it would not have been very satisfying.) This book starts out with Jason back in his world. He was a "missing person" for a while, then suddenly appeared in a cornfield in Iowa without any recollection of what happened. People just chalked it up to the concussion that he suffered before he went missing. However, he knew better. He went to the zoo one day to see the hippo and reminisce about the portal that took him to the other world. He heard music again, and this time jumped into the cage and forced the hippo to let him through the portal. From there he meets up with old friends, finds some new friends and goes on a quest.
Like most Brandon Mull books, Beyonders starts out in our world. A boy is studying biology, playing baseball and oggling a girl. He gets hit in the head which at first causes him to doubt what happens next. At the zoo where he works, he hears some music coming from the hippo, he looks and falls through the hippos mouth to another world. There he meats up with people, and sets out on a quest to find the syllables of a word that will defeat the bad guy. He also meets up with a home-schooled girl from earth who joins him on his quest. They also meet up with a "displacer" - a man who can remove and attach limbs at will. (This becomes quite useful for escaping from prison.)
Arcade Catastrophe continues the children's "realistic fantasy" story started in Candy Shop War. The world is a sightly fictionalized version of today's outer San Francisco suburbs. In this story, an evil magician uses an arcade to recruit kids to do his bidding. By accumulating enough tickets, kids can get "stamped" with a magic stamp that gives them special powers (like flying or swimming underwater.) They later use these in competition with each other to carry out the magicians tasks. The kids from the Candy Shop War get involved in an attempt to rescue the "magic police" who have vanished. There is a ton of action and suspense, with plenty of magic before the kids save the day and live happily ever after. It is not a bad story, but it is not as good as Candy Shop War. It is fun how he uses kid's favorite things (kids and video games) as the mechanism for magic. (Magicians know how to attract them.) I wonder what vehicle they will use for a third book?
Sky Raiders follows the typical Brandon Mull story arc. Some kids are living a normal life. Then, through in innocent action, they get thrust into a fantasy world. There they complete heroic adventures. Only unlike Fablehaven and Candy Shop Wars, they don't get to go home at the end.