Answer:
To eliminate Prospero's powers of sorcery
Step-by-step explanation:
Stephano must seize Prospero's books books because they are the source of his powers. The plan is not to destroy said books so that Caliban may use them once Prospero is dead. Without his books, Prospero is vulnerable as he won't have the power to summon the tempest and restore order, neither could he protect himself. This is the main reason why Caliban wants those books away from Prospero before killing him.