[The Ruling on Taking Zakat for One Who Owns Books Whose Value Exceeds the Nisab]

Answer—and Allah is the Granter of success: They have stated that an allowance is made for the scholar and the student of knowledge with respect to books he needs.
Based on that, the matter differs from person to person: the beginner’s need for books is limited; then the student who has passed the initial stage has a need for books greater than one below him.
Moreover, a scholar or a student may be passionate about research and reading, with a strong desire and inclination to expand in a particular kind or kinds of sciences—so an allowance is made for him in what he needs for that.
As for books that the student or the scholar does not need: they are not to be excepted for the poor person; if their value reaches the nisab or more, then they are counted against his wealth.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1