If I had a copy of Mark Wilson's Play Great Pool when I was sixteen I might have developed into a real pool player instead of an on-and-off-again banger. Even now, age 64, his book has helped me quite a bit, especially in areas like stroke and concentration.
Second and third favorites are tied between Dr Dave and Tor Lowry:
Most anything from the Dr Dave collection is valuable.
http://dr-dave-billiards.com/
Zero-X - Tor Lowry, his 'Compact Stroke" and teaching on aiming and patterns are priceless.
http://zerox-billiards.com/dvd-collection.html
Books
Pleasure of Small Motions, a real eye opener, by Bob Fancher
Banking with the Beard, never be confused about banks again, and learn some great one-pocket and bank pool banks, - Freddy Bentivegna.
Others:
There is some value in any book by Robert Byrne, but sometimes he packs too much into one or two pages, and the main idea gets diluted.
99 shots by Ray Martin - did not think much of this book at all, too primitive for today's pool market I think.