Tomes are books, scrolls and other writings obtained as Assets during a game of Eldritch Horror. Some may be initial equipment for characters, others may be obtained through an Acquire Assets action, and others still can be obtained through encounters.
Some Tomes function like other assets, like the Cultist's Journal, and give a straight boost to skills. Other tomes are used as actions, and usually involve a skill check, most often , in order to gain items like Spells, Clues, or other benefits.
Daisy Walker has a special ability that allows her to undertake the use of a Tome as a free action if she uses a Rest action. Agatha Crane may, once a round, undertake the action of a tome with +2 added to her roll.