Use Kimball's Biology Pages. A retired biology teacher at the high school and university level, Harvard graduate John Kimball created an online biology textbook so readers could easily find their way around the information. Having released several editions of his general biology textbook in print, Kimball based his site on the 1994 edition of his textbook, as well as adding newer, up-to-date information.
Visit the Estrella Mountain Community College website. Written by college lecturer Michael J. Farabee, the web pages provide a detailed view of biology at the college level. Having modified his lectures, Farabee has created a series of chapters covering topics from cells to biological diversity, complete with an easily accessible hyperlinked table of contents.
Utilize Rediscovering Biology's online textbook. Designed to complement the educational videos on the site, the online textbook provides extra notes and diagrams about topics including genomics and diversity. Unlike other sites, which allow you to view the information just in HTML format, here you can download the entire textbook or individual chapters as PDF files and save them on your hard drive for off-line use.