Books address a more specific topic in greater detail than other types of information sources (journal articles or dictionary entries). They are oftentimes a great source to find additional information sources, which are often listed in footnotes or bibliographies. This is especailly true of textbooks.
EXAMPLE
The first book below offers a history of the Four Spiritual Laws on pages 98-103.
Bill Bright and Campus Crusade for Christ by 
How Do I Get to Heaven? : Traveling the Romans Road by 