I already (IMHO) have a v. good teacher, and I feel he throws enough at me to keep me on my toes. The thing is, after reading the "Fun with Fives and Sevens" thread I got a nasty headache. Is there anyone who could recommend a good book on music theory? I feel I could make life easier in the future by getting a bit of groundwork done.
Ta.

If you want to hear someone 5's and 7's effectively, as well as some much more interesting licks, check out a song called "Lum-Lum" of the Greg Bissonette album, "Submarine".

Music theory is very important, IMO.  This is one of those things that is not 100% required to be a drummer.  But, like reading, has endless benefits.  There is really not a 'good' reason to avoid learning theory.  

That said, you are speaking of rhythm theory.  If you look at any theory textbook, you'll soon notice there is only one, maybe two rhythm chapters, and it  is normally about chapter 2.  This does NOT mean rhythm is less important than harmony, in fact, most theorists will tell you rhythm is more important than melody or harmony, simply because melody and harmony can NOT exist without rhythm.  I think the books are this way because, once you have a basic understanding of the math and of the symbols, rhythm is easy......


....in theory!!

Now all of that was a hair off topic, but i think is relevent.  To answer your original question, theory is a very important trick to have in your bag.  I have no idea how old you are.  If you are in high school or over 18, I would suggest taking some theory classes.  Some community colleges offer music theory.  Having a teach will make learning theory 100 times easier than doing it by yourself.  Until you can get a theory, or just to find out if you are interested in learning more, go to musictheory.net.  They have some good tutorials, explanations, and even some free training software that covers ear-training, interval and triad recognition, chord calculator, etc.