Due to the business my company is in, publishers often send promotional copies of books in hopes that we'll republish their content. Some of what we receive are technical books. Recently, as sometimes happens, our marketing department was good enough to give away books that are no longer needed -- whether they were used or not. I was lucky enough to score several technical reference books. One of them was Mark Sobell's A Practical Guide to Ubuntu Linux, second edition.
After just a short time, I'm already 120 pages in and have learned a lot about what makes the OS tick and what forces drive GNU and Linux in general. It has been an invaluable resource and is already helping me to better administer my desktop PC, where I have been using Ubuntu Linux for nearly two years (since just after the Feisty release). If you have the means, I highly recommend this book. It is well worth the while!