If you guys are serious about theory and harmony, this book is must have. I found out about it while I was at Berklee from professors and chair of Harmony department. Other professors who were performers recommend it to me and I got it without hesitation. I payed 50$ and it was one of the 50$ purchases I will never regret.
Read the whole book (it is very easy to read, follow and understand) and it goes from very basic to much more advanced things like reharmonization and other advanced concepts.
Here is a link I found using Google. Its on big sale, if you find it for cheaper than that price, please let me know!