Books donated to a library. The term also sometimes refers to gifts of all publication types.
Donors of potential library materials are usually offered a "gift in kind" receipt for tax purposes. This receipt also shows that donors receive no special treatment for having made a donation. (Unlike financial benefactors, who may receive anything from a custom bookplate or brick to having portions of the library named after them. Paid Friends of the Library members may also receive additional services.)
Many book donations are unsolicited, and in several cases, unwanted. Unwanted items may be sold or disposed of as the library deems fit. Donors may request gifts back if its deemed unworthy of being added to the collection.
As part of a sound collection development policy, most libraries will refuse conditional gifts. This avoids an outside influence from shaping the collection. There are many individuals and organizations focused on shaping library holdings -- manifested by either donating books expousing a certain point of view, or stealing/destroying those that don't.