In my profession it's important to understand the implications of various licenses, beyond "GPL is viral" and "Apache is fine". This book was recommended to me by a co-worker, and it does indeed a great job in explaining the various intricacies of various licenses. While the material can be a bit dry sometimes, the book is very readable and I actually enjoyed reading it.

Obviously, reading this book doesn't mean alleviate the need for a corporate lawyer but it gives the reader a solid introduction into the area of open source licenses and what they try to achieve. It's interesting how the different licenses handle software patent issues and to see the differences in the patent defense approaches.

The book is written in 2004, so it makes no mention of the new GPLv3 license and more recent development of software patents as they relate to open source.

It is actually available on-line as well (http://www.rosenlaw.com/oslbook.htm), appropriately under an Academic Free License.