first is the classic Hanson book "C Interfaces and Implementations", http://www.amazon.com/Interfaces-Implementations-Techniques-... while you may not be coding in C the basic paradigms do carry over.
second is use the API a couple of times and you'll see natural cleavage points.
i generally develop APIs on paper first, which also helps me ensure consistency that is absent when i develop them too quickly or organically. i keep in mind my goals, i document as much as possible, and i try and stick to a paradigm.
hope that helps. i've written APIs used in internal services and applications but never externally, publicly available. i'm no expert but that's how i start.
do it, use it, learn, evolve.
it doesnt matter how much do you read about.