Books: Everything Scrabble
While the millions of Scrabble sets sold throughout the world come with cute little rules printed on the top cover, the real authority on Scrabble is the book "Everything Scrabble," a fairly comprehensive work covering just about 95% of the skills you need to play Scrabble at a high level.
The book is divided into four parts - the first part details basic strategy (master this, and you'll consistently beat casual players), the second part goes into advanced strategy (good advice for anyone making the jump into club play), the third part presents more Scrabble puzzles than you can shake a stick at, and the fourth part details Scrabble tournament play and some notable games.
Scrabble is one of the few board games that tests a wide suite of abilities: pattern recognition, mathematical calculation, spatial visualization, probability estimation, and memory (vocabulary, to be precise). There's even a mild bluffing mechanic - unless you're playing a computer, it's possible to play a phony word and get away with it, depending on who your opponent is and how he plays.
The only real downside to this volume is that it's already a little out-of-date. This also brings up a larger issue, one endemic to the game itself. Competitive Scrabble, for all its virtues, does not change as frequently as the English language does, which is why words like "texting" and "internet" are not valid in the tournament word list of 2006. Strange, no?