This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori 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.