Shojiro Sugiyama's 25 Shoto-Kan Kata is the clearest and most concise book on the Shotokan katas. Every black belt I know carries a copy in his gym bag.
In preparing this forum, my colleagues and I used Sugiyama's book as the standard reference for numbering the moves of the katas. That way we could refer to "step 17 of Heian Nidan" and know that we were all looking at the same technique. If you want to follow along, you'll need a copy of Sugiyama's book. If you are a Shotokan, you should own it anyway. Buy it.