Dd loved this book. It's simple, silly and sweet, and the child doesn't look particularly like one gender or the other so it's good for either.
The last page is just the word "undies!!" and all different fun looking pairs. It was a great bit of incentive for dd as it was the first time she seemed excited about getting her own undies.
The other trick we used was what we called a "well done box" - a shoebox decorated with fun giftwrap, filled with cheap little treats, like stickers, toy cars, new crayons, small colouring book, plastic animals and dinosaurs, and party favour type stuff. At first, she got to choose something from the box every time she successfully used the potty. The goalposts slowly moved as she progressed - after awhile she got to pick something from the box for using the big toilet when we were out and about, or for keeping her undies dry for a whole trip into town, or for a full dry / clean undies day, etc... whatever she was finding a challenge at the time. We made a bit of a ceremony of presenting her with the box and wondering aloud what she would choose, and she really loved it. She'd start planning in advance what she was going to choose 'next time' so it worked well for getting her to think about using the toilet before she urgently had to go.
We kept re-stocking it for as long as it helped, about three weeks in all (and really, she didn't accumulate much more treat-wise than she would have anyway, they were all small things most little kids would have anyway), and when she didn't really need it anymore we let it come to a natural end when all the treats were gone. I was a little worried about how we'd finish with it, but we just started acting all excited about how close she was getting to 'getting all the treats' - she was proud instead of upset to get to the end of it.