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.