In my humble opinion, the best book in the whole wide world for music theory is (unsurprisingly), the Associated Board's book. It's called
The AB Guide To Music Theory Vol. 1 and every music shop that sells sheet music and/or books should have copies for sale (or buy it here:
http://www.amazon.co.uk/exec/obidos/ASIN/185472446
0/ref=pd_sr_ec_ir_b/026-2114671-5326033).
There are two versions - Grades 1 to 5 (which is pink) and Grades 6 to 8 (which is blue) and it's only a fiver. AB, by the way, is short for ABRSM which is short for Associated Board of the Royal Schools of Music and is easily the highest-acclaimed music body in the UK.