Before Borders closed down here in York I saw they were selling a manual for Ubuntu. The book was thick enough to use as a door step.
If a friend encouraged you to try Ubuntu you will already have found out how easy it is to use and work with. However, when you have the odd question you would previously have found that instead of phoning your friend, the answer to your problems could have been found by simply putting your problem in a search engine or you could have tried the Linux community forums that would have quickly provided the answer.
Now, for those who prefer a book, there is now an Ubuntu manual being compiled at http://ubuntu-manual.org/.
If you want to contribute to the writing of it they are inviting people to get on board. The release is timed to coincide with the release of the new Ubuntu out at the end of April.