藏书 is a separable verb (离合词) meaning 'to collect books'. The verb 藏 and the object 书 can split when you add quantity, duration, or other elements between them — for example, 藏了不少书 ('collected quite a few books'). It is also commonly used as a noun to refer to a book collection (e.g., 他的藏书很多 'his book collection is large').