For the most part, books are generally thought of more as secondary sources than primary sources. But oftentimes books can be used as primary sources. For example, let's say you're writing about the 19th century naturalist John Muir and his views on the environment, you could use some of his writings and books as primary sources.
In that case, you could just do an author search for Muir, John. On the other hand, books about John Muir that were written by other authors would be used as secondary sources.
For places to find books, go here: Secondary Sources