The issue of open source and free software licensing is commonly referred to in many contexts these days – but often without a good understanding of what the differences are between the various open source and fee software licences. Andrew M. St. Laurent’ book, Understanding Open Source and Free Software Licensing, provides some useful background, and can be read online for free – all chapters are available as PDFs.

Chapter overview as follows:
Chapter 1: Open Source Licensing, Contract, and Copyright Law
Chapter 2: The MIT, BSD, Apache, and Academic Free Licenses
Chapter 3: The GPL, LGPL, and Mozilla Licenses
Chapter 4: Qt, Artistic, and Creative Commons Licenses
Chapter 5: Non-Open Source Licenses
Chapter 6: Legal Impacts of Open Source and Free Software Licensing
Chapter 7: Software Development Using Open Source and Free Software Licenses

(Thanks to Jane for the reference, and to the O’Reilly Open Books project for making it available)