I highly recommend: The Everything Potty Training Book by Linda Sonna:
Why: It has several methods included, so you can choose the one that fits you best, or combine elements from several methods.
It is research-based... you are not just getting someone's personal adventure.
It has awesome troubleshooting tips that really work.
My experience: With my 3 yr old son, we adopted one of the slower methods, the "practice" method, combining elements from the "fast-track" method that seemed to fit our schedule. All went well until accidentally one day my son pooped in the bathtub while he was taking a bath. Unfortunately, he decided that this was very comfy, and it got a great reaction out of mommy. So the next day he did it again. And again. And again...
The central lesson of this book was... no negative reactions, no matter what happens, ever... Positive feedback only. I tried. I tried rewarding him for using the potty instead of the tub. The reward offers got bigger and bigger, and turned into massive bribes. No dice. Pooping in the tub was just too fun. What was a mama to do? If I started punishing him for pooping in the tub, I risked messing up all the great progress we had made so far. But I absolutely could not stand cleaning up that horrible mess one more time. No way. I was about to lose it.
So I went back to the book. And I found a little story about something Dr. Sonna's mama had said once, about pooping in pants. "In OUR family," she had said, "we do not poop in pants. We poop in the potty." Well, I thought, I'll try. So the next time my son was about to get in the bath, I said to him "Son, there is something you should know. In OUR family, we do not poop in the bathtub. We poop in the potty." I said it very proudly, like the Queen of England. His eyes got very big. Clearly he had not thought of this as a matter of family pride. He never pooped in the bathtub again. Sometimes I had to issue reminders. Once, I added, "I don't know about those OTHER families, down the street. But in our family..." Worked like a charm. My husband never got the hang of saying it with a straight face, though.