Shapechanger. The spellbook mimic can use its action to polymorph into an object 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 can adhere to anything that touches it. A Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 13). 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.

Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage. If the mimic is in object form, the target may be subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 4 (1d8) acid damage.

The spellbook mimic can use 3 legendary actions, choosing from the options below.  A legendary action can only be used according to their specific properties for each action. It regains all of its legendary actions at the start of its turn.

Spellwrite. The spellbook mimic can create or remove any spell written in its text for a turn at 3rd level or lower (including cantrips). This restriction can be mitigated if the spell information comes from a separate source (either a companion or direct source) This can only be done once per turn.

Spellcast. The spellbook mimic can cast any spell written in itself. Any spell that would require an action or bonus action to cast can be cast during its turn, expending a legendary action. Any spell that would require a reaction to cast can be cast as a reaction at any time expending a legendary action, provided a legendary action is available. Its spell save DC is 15, or its wielder (if applicable) depending on which is higher.