The reason we use the “best books” or "best science fiction books" motif is that 95% of book searches on Google include that phrase. And, instead of fighting that huge trend we decided to go with it. In user testing, we found that users think that way when looking for a book in a bookstore as well, even though they don’t mean “best” objectively but rather someone's favorite.
I know this makes some authors nervous, but please understand this is not a literal phrase, rather a subjective one. I think this trend is due to the internet/Yelp effect. As when you ask someone where to find the "best Thai food" you don't mean objectively the best Thai food in the world, you mean their favorite.