It got mentioned on the last OpenJavaDay (thanks Ibon!) with such a good review that I got my own copy of Practical API Design by Jaroslav Tulach. The book describes the set of standards developed by the Netbeans team during the last ten years, from framework novices to their current state of maturity.
Even if you don't agree with some of the book recommendations (which is ok, since they aim at being a bit controversial) it is still a complete source of good practices when designing - not an application, but - a platform. If your case is not as much about "design" but "design while ignoring how many people are using your API and which parts are found useful", this book is a must-read.
If I had to complain about something, it spends too much time in the introduction chapters. It sets the foundation stone of good API design and I really needed to grab some perspective about API stability over time, but a bit shorter would have made it perfect.
I love the freshness in the style, the way it seems to be designed with my case in mind. A quote after several pages describing its intended audience:
If you are still asking yourself wether you should read the book or not, there is a much shorter answer: "Yes, you should read this book!"