There are several books available for beginners who are just beginning their adventure into the land of WordPress. Two of the best books specifically for WordPress are:
Digging Into WordPress by Chris Coyier & Jeff Starr
Digging into WordPress is a practical guide full of information that you can put to use immediately in your work with WordPress. Not only is this book a great find, but the website, http://digwp.com/, is an ongoing blog with helpful hints in the world of WordPress.
WordPress for Dummies by Lisa Sabin-Wilson
Lisa covers vital information on both WordPress.com and WordPress.org in her book, “WordPress for Dummies”. She does a great job starting with the beginning, but including more intermediate topics such as beginning concepts in PHP and CSS so that more advanced users can get their hands dirty. She also has a companion website that accompanies her book and includes bonus chapters.