Ideally, you should first practice long straight in shots for hours and hours before you practice anything else. Your cue ball should consistently come to a dead stop with no side spin. Otherwise, there's something wrong with your stroke that will carry over to any other drill that you do, and you won't have any consistency anywhere else. Any flaw in your stroke will show up here.
There are a number of books and DVDs that have excellent drills and shots that will help improve your game.
If you were to buy only 1 pool book for the rest of your life, you should buy "The 99 Critical Shots of Pool" by Ray Martin. If you practice the shots and read the section on fundamentals, this will really improve your game. It's real cheap too.
http://www.amazon.com/99-Critical-Shots-Pool-Everything/dp/0812922417
I agree the ray martin 99 critical shots is an amazing book. I've learned a huge amount from that book, even though it isn't a drill book. Az has a few good drills that I found. Force follow on youtube is a good video guide, there are tons of books. Personally i've been using the circle and the madness drill as my two primary practices.
Another stroke drill to ensure you're hitting center ball is the rail trick. You go from middle diamond to middle diamond but never raise the cue from your stroke. If hit with proper center english [and your playing on a proper equipped table] the cue ball will hit the middle diamond on the far rail, and return in a perfectly straight line to your cue tip. Otherwise just check out some material from some of the instructional guys.
FWIW hope it helps.