I have bought several game guides from BradyGames, Prima and PiggyBack but none have exceeded my expectation except Future press.

Future Press has provided excellent content in their game guides: Killzone 2 & 3, Portal 2, Bayonetta, Infamous 2, Deus Ex, and Vanquish.

I'll be purchasing their upcoming game guide, Dark Souls.
Amazon.com: Dark Souls The Official Guide (9783869930527): Future Press: Books