The Enchanted World of Sleep is an interesting look into how sleep works, and the science behind sleep studies. The book doesn't promise to have all of the answers to why living creatures need to sleep, but rather presents a history of the various beliefs and scientific breakthroughs in understanding the cause and nature of sleep.
Both anecdotal and factual cases are presented, and the book touches on a variety of sleep-related subjects, including dreams, sleeping disorders, the reasons why our bodies need sleep, whether or not it's necessarily true that our bodies need sleep, the effects of sleep deprivation on humans and animals, the myth of needing 8 hours of sleep to be healthy, past beliefs about sleep, and a glimpse into how and why our bodies fall asleep.
The only real problem with the book, as with any scientific journal like this, is that the information is dated. The book was published nearly 20 years ago, so don't go into the book expecting an up-to-date account of what we know about sleep.
However, as a history of sleep, the book isn't terribly dated.