There is a new and seemingly very good book about Scrabble (in PDF):

http://www.scrabbleplayershandbook.com/

From the introduction:

This book has been written by a dozen of the world's leading Scrabble players, all of whom have
competed in past World Scrabble Championships and many other major international tournaments.
Our intention is to provide an easy-to-read guide to everything you need to know about this
wonderful game, whether you are a regular online player, perhaps thinking about stepping
nervously into a Scrabble club for the first time, or already a seasoned tournament attendee
interested in improving to expert status. We'll start off with the basics of competitive Scrabble and
end up covering everything the game has to offer at all levels.

The Scrabble Player's Handbook is available free of charge. There is no copyright on the contents and readers are encouraged to distribute the book in PDF or printed form to all who would benefit from it.