When referring to a thing (not a person) use the relative pronoun which to intoduce a non-restrictive clause.

Example: The librarian organized the books, which were written by Robert Cormier.

When referring to a thing (not a person) use the relative pronoun that to introduce a restrictive clause.

Example: The librarian organized the books that were written by Robert Cormier.

The two previous examples may seem similar, but have different meanings. The first example, the non-restrictive clause, means that the librarian organized the books and all of them happened to be written by Robert Cormier. The second example, the restrictive clause, means that of all of the books in the library, the librarian organized only those that were written by Robert Cormier.