The problems many writers have with the words that and which make copy editors inveterate which-hunters. Remember that a nonrestrictive clause is not essential for the reader to understand the full meaning of the word or words that it modifies. It simply adds more information, describing but not limiting (“restricting”) what it modifies. On the other hand, a restrictive clause contains information that is essential for the reader to understand the full meaning of the word or words that it modifies. It limits (“restricts”) what it modifies. To keep things simple, use the relative pronoun that to begin restrictive clauses and which to begin a nonrestrictive clause. Examples:
He showed me the book that arrived in the mail today. [The meaning is restricted to just one book—the one that arrived in the mail today.]
He showed me the new Stephen King novel, which is the one I told you about yesterday. [The clause just adds more information to the sentence.]
Also note that you can often delete the word that in many constructions: The books [that] I ordered arrived today.