Lately, your Guru has been fielding this particular question quite often; it’s high time we cleared this up – and again, it’s easier than you probably think it is.
Let’s start with a simple sentence. Which of these is correct?
A. The book that your Guru wrote is about grammar.
B. The book, which your Guru wrote, is about grammar.
Well, both are correct. Your Guru could give you the long, boring answer (probably the one
your third grade teacher tried to pound into you), but she’s here to help you, not lecture you.
So, just ask yourself: Do we need to know that the Guru wrote the book, or is that information incidental? If you are identifying the book by telling us that the Guru wrote it, you should use “that.” “That,” in this instance, is restrictive, meaning that it’s restricting the subject to something specific.
If, however, your main point is the book, and it is only incidental that the Guru wrote it (and
when is your Guru incidental? She hopes you don’t answer that), you should use the more non-restrictive “which.” And, of course, don’t forget to set off the phrase with commas.