Is selling knowledge or academic books forbidden?

Whoever is asked about knowledge he possesses, it is not permissible for him to conceal it. However, it is permissible to take a wage for teaching religious knowledge, or a fee for issuing a fatwa or judging, if the Mufti or Judge does not have sufficient income. Similarly, it is permissible to sell religious books, because what is being sold is the paper, ink, and effort expended, not the knowledge itself.