In so saying, he confused poor Flora. The reason for her confusion is that the example is incorrect. “Book of poetry” is modified in this sentence as a single entity … in effect, as though it were a single word instead of a phrase. The use of “which” in the second sentence above is incorrect unless a comma is inserted after “poetry”, to wit: “I read the book of poetry, which came out last year.” In other words, you are adding information about the only book in question – which is what we were saying all along.
Sorry, Chef Troy, 'tis you who are incorrect. In my example, “book of poetry” was not to be taken as a single noun. “The book of poetry that was published last year” means that the poetry was published last year. “The book of poetry which was published last year” means that the book was published last year. This is an exception – and a fairly useful one – to the general rule that “that” goes with dependent clauses and “which” goes with independent clauses.
Or, to put it another way: When you have a dependent clause (" . . . that was published last year") following a phrase containing more than one noun (“book of poetry”) and referring specifically to the former noun and not the latter, the word “which” is drafted to stand in for “that,” because “that” would refer strictly to the last noun before it (“poetry”).
It’s a little-known rule, and I only learned about it myself a few years ago, from the grammar column in the Atlantic.
[ul][li]The book that was published last year: As opposed to the book that was published this year.[/li][li]The book, which was published last year: We have been talking about a book, and I am now adding that it was published last year.[/li][li]The book of poetry that was published last year: That is, poetry that was published last year, contained in a book. As opposed to poetry that was published this year, contained in a different book.[/li][li]The book of poetry which was published last year: That is, a book that was published last year and contains poetry. As opposed to a book that was published this year and contains poetry.[/li]The book of poetry, which was published last year: We have already been discussing a book, and it was published last year. (Under other circumstances, “which” might also refer to the second noun, which in this case is “poetry,” and the phrase would be ambiguous. But since poetry is published all the time, “poetry, which was published last year,” is a silly phrase, so we intuitively understand that “which” refers to “book.”)[/ul]