Do you want nonfiction books with information about cats? Or fiction books with cats as characters or part of the plot?
If you mean the latter, The Cat Who . . . series by Lillian Jackson Braun is a cute, cozy mystery series. Although each separate book is listed at $7.99, the first three have been combined into 1 volume for $12.99, making each title less than $5.
My favorite information/reference book about domestic cats is Cat Watching by Desmond Morris. It is currently not available at B&N (?) .