i believe the best way to buy books, is to buy by needs.
what problem does the book intend to solve for you or what knowledge are you seeking to acquire at that specific time in your life. if you buy by needs, you will sure have the motivation to read and digest the book's content because every word is a piece of the solution you are looking for.
However for those that are buying because they just love reading, their favorite genre should be used as the buying criteria.