I've been doing a lot of reading about open source licenses lately. There are many books and articles about the "open source phenomenon" but to my knowledge there are really only a handfull of books that explore the nitty-gritty of actual open source licenses. Two I've read recently:
Lawrence Rosen's meticulous Open Source Licensing: Software Freedom and Intellectual Property Law was published in 2004 and is available in print or in PDF segments. I found the hard copy worth buying - it is well-thumbed and full of highlighted areas now that I've read it straight through then come back to reread sections as detailed questions pop up. This is a dense book, and it doesn't provide black and white answers. However, it is worth the effort.
Understanding Open Source and Free Software Licensing, by Andrew M. St. Laurent was published in 2004 and is also available in print or PDF. This book jumps more quickly into the licensing terms of specific licenses, making it somewhat less valuable as an overview book but a bit better for examining the fine details of some licenses. It also covers the Creative Commons licenses and goes into some detail about why shared source licenses are quite different from open source licenses. When you are using one of the big open source licenses (GPL, BSD/MIT, Apache, MPL) and need a solid reference, this book does the trick. It is well-written, clear, and well organized.