For a very deep, thorough and painful treatment instead of feel-good books, read "Practical API Design" by Jaroslav Tulach. Yes, it's Java, but it exemplifies the fundamental tradeoff that the feel-good books ignore: the more powerful you make your API for users, the less potential for evolution and long-term maintenance your API retains.
RESTful Web APIs (http://shop.oreilly.com/product/0636920028468.do) was a good starting point for understanding RESTful HTTP APIs.