I am often asked to explain the different open source and free software licences. This book, by Andrew M. St. Laurent, can be read online for free. All chapters are available as PDFs.
And for more free online books, take a look at the O'Reilly Open Books Project. It also includes The Whole Internet User's Guide & Catalog - now out of print, this was the first comprehensive user guide to the Internet. It sold over 1,000,000 copies and was named one of 100 Books of the 20th Century by the New York Public Library.