Both are correct, similarly used, and understood, but "about" is proper and "on" is colloquial.

They are very similar. A book that is "on" something usually has a clear topic. For example, a book on fixing boats. A book that is "about" something is usually broader. For example, a book about the Amazon. However, they can both be used interchangeably.

Thanks, James.

They are interchangeable, but "a book ON something" makes for better puns :)

Edited, see my answer.