I've had some downtime recently to get through my list of "read it later" bookmarks. This InfoQ article from last March gives an overview of a short ebook published by Apigee's Brian Mulloy called "Web API Design - Crafting Interfaces that Developers Love" (downloading the ebook requires signing up for an Apigee account - free of course). It's a quick read at 38 pages and covers the basics really well. The book also does a good job of avoiding too much theory by using API examples from companies like LinkedIn, Facebook, Twitter, Foursquare, etc (the Digg examples were awkward though). Like me, chances are you already know most of this stuff, but it was still useful to revisit the basics and validate that the stuff I'm building at the company I work for fits most of the best practices outlined in the book. I even managed to learn a few new things.
For beginners, I highly recommend. For veterans, it's still good, if only to remind yourself of the basic best practices.