Is it permissible to pray behind a mosque imam who sells books by atheists in his private library?

It is permissible to sell books authored by atheists if their content is beneficial or permissible, and prayer behind the seller is valid. However, if the books promote atheism, it is obligatory to inform him of the danger therein. If he insists on selling them out of greed for money and does not approve of disbelief, he is not ruled a disbeliever, and prayer behind him is valid, though disliked. Praying behind someone else is preferable, and his case should be referred to an authority to stop him.