Richard Best has invested a lot to understanding the GPL, inside and out. He now has an ebook to help guide others on the GPL.
The content of the book looks pretty interesting, though I haven't yet made the time to get through it. I'm not sure how many folks will spring for this, as it's quite a niche book topic, but I hope for Richard's sake that it does well, and I'm sure those that do read it will gain a good bit of value.
Also valuable from Richard recently, is a human readable version of the GPL that he wrote.