I am very much a fan of the Take Control ebooks from TidBits Publishing. These are ebooks in .pdf format (though they are also available as printed books) designed for people who want to start using Macintosh hardware and software right away to get things done without wasting time looking for the information you need. Take Control books are well-written, accurate, and very easy to use because they're designed for easy navigation. The table of contents, as well as internal references, are all hypertext links. The ease of use begins with the Quick Start page, which lets you find exactly the sections you need to read to do what you need to do, right away, and go there with a single click. Plus, if you register your ebook, and an update to the edition of an ebook you buy is released, you can simply download the updated version.