Yes it is, but unless a reference is a website isn't every book reference meaningless as you say unless someone owns the book. I thought the point of the reference was to provide validity to the entry.
To be clear, I don't object to adding the reference, I just wish authors would keep their item numbers unique; a couple of town letters before the number (like atwood-coffee). As the numbers also allow you to find items within a reference book, so would be nice to avoid a long list of Gross# 1.