Books cover topics in depth. Compared to articles, they usually cover more ground and provide more background information.
Look through the bibliographies of books to find further reading.
Is it up-to-date? For my project, how up-to-date do my sources need to be?
Who is the author? For my project, is it necessary that the book be scholarly? (Scholarly books are written by scholars for other scholars).
For more, see the Library Guides page on evaluating sources.
Dietary Components and Immune Function by 
Professional Writing in Kinesiology and Sports Medicine by 
