Hallucinations

There are a number of different neural conditions that can cause hallucinations. Different senses can also be involved. Visual hallucinations are well documented. However, sound and smell also occur. People may be reluctant to admit to "hearing voices" for fear of being branded crazy. Some medical conditions such as migraines and epilepsy may be associated with hallucinations. They can also be triggered by drugs (including recreational ones as well as others lake that used to treat Parkinson's disease.) The author of Hallucinations did some self-experimentation with various drugs in the 50s, but found he had to stop before his "experimentation" started creating problems in his life.
Hallucinations are usually outside the control of the person. Somebody may see people multiple or hear people where there are none. With some, it is common to see hallucinations based on past experiences. Musical notes appearing randomly seems to be a fairly common manifestation for those that spend a great deal of time reading music.
Some religious experiences may be scientifically explained as types of hallucinations. (If God works through natural means, then this could be a logical means that are used.) A final type of hallucination covered is "false limbs". People often "feel" a limb still present even after it has been amputated. However, when they start to lose other sensual confirmation, they gradually lose control of it. This can be counteracted by seeing visual confirmation of the activities (by a prothesis or other tricks.)
The book has a somewhat random assortment of stories and descriptions of cases where people's senses show them something that is not actually there, and seems to be written primarily to entertain.