Q. Are books primary or secondary sources?

The short answer to this question is, it depends. 

It depends on the content inside the book.

For example, if you were researching Mary Shelley's novel Frankenstein...

• the book Frankenstein (the work of literature) would be a primary source.
• a book of literary criticism about Mary Shelley's novel Frankenstein would be a secondary source. 
• a book containing Mary Shelley's letters would be a primary source.
• a biography of Mary Shelley's life would be a secondary source. 

It depends on the context of the source in relationship to what you're researching.

For example, if you were researching global warming...

• a book about global warming that was written during the 1990s would be a secondary source.  

If you were researching how people talked and wrote about global warming in the 1990s...

• the SAME book about global warming that was written during the 1990s would be a primary source.