CalifJim wrote:

That is restrictive and and tells the reader which one you mean. ... Which provides additional information and the clause is set off by commas. So it is your contention that which is always non-restrictive and always requires the commas associated with non-restrictive clauses? CJ

No, I'm sure there are examples of non-restrictive uses that don't require the commas. But as you know, if it's a "parenthetical comment," it can removed entirely from the sentence, and I would argue that if you can do that, then you DO need commas on either end of the "which phrase."  But I know you have a lot more linguistic training than I do, so I'm willing to bet that you have a lot more examples.

And like I said also, I'm pretty sure that the which/that disctinction is going the way of who/whom, and there is not going to be any difference in another couple decades.

Believer wrote:

The book [which you told me to read, by the way] was very good. The part in brackets (???) are not JUST additional information but essential information that gives restrictive meaning to the sentence.  Which book? the book you told me to read and thus, should not set off aside as an element (piece) of additional information.

That wasn't how I intended it - hence the "by the way."

Did you see the movie Ella Enchanted? It was just terrible! But the book, which you told me to read, was very good.

The "which you told me to read" is unnecessary for the sentence to work. It's just additional information that you can lift entirely out of the sentence.

If I wanted "the book recommended by you " to answer the question "which book?" then I would have said "The book THAT you told me to read was very good."