This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
You normally focus on specific use cases that you would like your API to be able to support. One good book on API design is "Practical API Design" by Tulach - it's a bit dated but I think the material will still be relevant.