Anyone who has read these books, can you tell me the pros and cons, what you thought of it, whether it was easy to comprehend, things like that. Or suggest a different book..

Harmony by Walter Piston
Theory of Harmony by Schoenberg
Tonal Harmony by stefan kostka and dorothy payne

I used the Piston book in college and didn't really like it. It was pretty dry as I remember.

Piston's book did not go nearly as deep into harmony as Schoenberg's book. Problem is, it is not a cakewalk to get through; filled with extraneous but interesting thoughts.
The third book you mention I do not know.
Harmony is important for a bass player ( jazz ) to know something about.
Compare the minor mode of Schoenberg to Piston; ditto with modulation.
No comparison.

i found the tonal harmony book the best of the three to go to with a specific question in mind. its got a good index and is easy to use. the piston is a good teach yourself harmony book if your willing to go through it slowly. the schoenberg harmony book has only sat on my book self... but it makes me look smart, i guess. the kostka book is probably what your looking for.
chris

I've taught theory using the Kostka-Payne book for the past seven years. It's not perfect, but it is very good, and I like it better than anything else out there that I've seen.