Conversations on the Plurality of Worlds by Bernard le Bovier de Fontenelle translated by William Gardiner
This book is a mixture of science and science fiction. It explores the various bits of astronomy that were known in the 17th century, and then speculates on the other species that inhabit planets and moons. The author suspects that the moon and other planets have inhabitants. They observe the earth in a way similar to how we observe the moon and other planets. It may have been written as pure science, but could easily read as a modern science fiction book.
