The Initiate: Some Impressions of a Great Soul (1920)
by Cyril Scott
Wikipedia, but I haven't had time to read the whole thing yet, and I doubt most would consider it to be a book. It can be downloaded and kept offline, like a book.
I once read that if you tried to read every Wikipedia article for 12 hours a day, it would take you about 42 years to finish—assuming no new content gets added along the way.