Spellbooks, ever since the Summer Update 8.2 have provided a source of defense from attacks, as well as showing spells, and best of all, raising a mage's magic level by different amounts.
Spellbooks are one handed and go in your shield slot, however they can go in the arrow slot but no affect will occur on your magic level. 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 seven spellbooks in game so far, they are: