Considering to buy "Theory of Poker"

Both books are a great read but may be of little use to you until you get to limits of $6-12. Most low-limit players can be beat by anyone who plays solid straightforward poker. If you need to make a choice, go with TOP. There's a lot of great info in Advanced Hold 'Em, but most of it is useless at the limits you're at, because the actions suggested in this book are lost on your competition.

Have you tried Lee Jones' Winning Low Limit Hold'em? It's an excellent book for the limits you are playing.

Good luck.