I agree, the Seirawan books are definitely good. Silman is *fantastic* for more of a beginning intermediate player.

It's never too early to start tactical training--you can pick up a book of exercises ("Combination Challenge" by Hall is a beginning intermediate level, maybe "1001 Winning Chess Sacrificies and Combinations" by Reinfeld for more of a beginner). The Seirawan book on tactics is an excellent introduction, and might be a good way to prepare for the straight diagram challenges.

You'll see a lot of his work at the bookstore, I would say that in general I don't find books by Bruce Pandolfini (of "Searching for Bobby Fischer" fame) to be particularly helpful, although they certainly aren't terrible.

"Comprehensive Chess Course" by Alburt is a very, well, comprehensive introduction. The first two volumes will really help you develop your visualization skills and lay a strong foundation for further development. The other volumes in the series are great introductions to their topics but may be a little incomplete by themselves.

Hope that helps!

John