Book Golem

Immutable Form. The golem is immune to any spell or effect that would alter its form.

Constructed nature. The golem does not require air, food, drink, or sleep.

False appearance. If the tome golem is motionless at the start of combat, it has advantage on its initiative roll. Moreover if a creature hasn't observed the golem move or act, that creature must succeed on a DC 18 intelligence (investigation) check to discern that the tome golem is not an ordinary stack of books and scrolls.

Multi-attack. The golem makes two slam attacks.

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft, one target. Hit: 6 (1d6+3) bludgeoning damage.

Spell-casting. The golem cast one of the following spells, requiring no material components and using intelligence as the spell-casting ability modifier (spell save DC 14).

1/day: Overwhelming knowledge. The golem telepathically reads the content of various books and scrolls composing its body to a target it can see in the span of mere seconds. The target must succeed on an intelligence saving throw and suffer 8 (2d4+4) psychic damage on a failed save, or half as much on a successful one. On a failed save, the target has disadvantage on intelligence checks for the next hour. 

1/day: Literary trip. The golem can enter a nearby bookcase and choose another bookcase it can see to come out of next round. While in a bookcase, the golem has total cover against attacks and other effects outside the bookcase. If the exit bookcase is destroyed between the two rounds, the book golem exits prematurely from the destroyed bookcase and suffers 2d10 damage.
The bookcases in the room appear normal while the golem is hidden in it. A creature can succeed a DC20 Investigation check to notice a glyph activating on the exit bookcase.
At the start of its next round, the golem regains 1d8+4 hit points. It then bursts out of the exit bookcase and has advantage on his first attack towards any creature within 10 ft of the bookcase that failed on their investigation check.

Arcane absorption. When targeted by a spell attack, the golem can use its reaction to absorb the incomming spell into one of its books. Roll a d4, on a 1, the golem succesfully absorbs the spell, the spell has no effect and the golem regains 1d4+4 hit points.