Llengrath's Martial Masteries is a grimoire in Pillars of Eternity II: Deadfire.
A grimoire is the lifeblood of any wizard's magical repertoire. These books are painstakingly constructed with rare materials arranged in precise configurations, and are able to temporarily hold fragments of ambient soul essence, which is channeled through the book and discharged in the form of a spell.
It is a popular misconception among the uninitiated that wizards are the shapers of the magical energies they wield. In truth it is the pages of the grimoire that do the shaping. The art of wizardry is in the disciplined control and manipulation of the grimoire, with the most skilled wizards able to coax spells of maximal effectiveness and magnitude from their tomes.
Though all grimoires are limited by a finite set of pages, many accomplished wizards are known to carry several grimoires at once to handle a variety of situations.