Shapechanger. The mimic can use its action to polymorph into a bookcase or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only). The mimic adheres to anything that touches it. A creature adhered to the mimic is also grappled by it (escape DC 14). Ability checks made to escape this grapple have disadvantage.

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary object.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it.

Living Library. The mimic has 10d10 + 10 books upon its shelves. Each book is a juvenile mimic, which remains dormant until ejected by the mimic's Book Throw.  An ejected book is replaced when the mimic finishes a long rest.

Pseudopod. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) piercing damage plus 9 (2d8) acid damage.

Book Throw. Ranged Weapon Attack: +6 to hit, 20/60 ft., one target. Hit: 9 (2d4 + 4) bludgeoning damage, and the target is grappled by the newly ejected juvenile mimic (escape DC 5).  If the attack misses, the juvenile mimic lands in the nearest unoccupied space to the target.  Juvenile mimics created this way take their turn immediately after the bookcase mimic.