Spellbooks can show your learned Spells. Ever since the Summer Update 8.2, they also provide a source of defense from attacks, and best of all, can raise a Mage's Magic Level by different amounts.
Spellbooks are one-handed and go in your shield slot. In relation to normal Shields, spellbooks have very low defense, so may not be as attractive to people on a solo hunt, but better used in a team hunt where the mages don't have to take hits.
There are eight spellbooks in game so far, they are: