Here's a new reason to be wary. A colleague of mine sent me this exasperated email about a querier who used comp titles:
He can't have read any of the specific books, two of the authors hadn't been published yet and one of the books will have a different title when it IS eventually published. And I may be the ONLY thing those books have in common.
Notice the different title reference?
If you use Publisher's Marketplace, or even Amazon, to cull titles for comparison, be aware titles change, and sometimes they change AFTER they're on Amazon or BN.com or other lists.
You don't need a comparison, but if you just can't resist, use books you've read. ***
There are all sorts of pitfalls for the unwary querier but this one can be avoided pretty easily. As such, when I see this mistake in a query, I draw conclusions you're not going to find flattering.
***an interesting and valuable question from the comments
Just a thought - but I'm on a couple of writer's forums and a lot of us have read each others arcs and manuscripts. I would have assumed that the querier had read the books before they were published - but I've also been accused of being a bit on the naive end of things.
Would you say it's bad form to compare your book to pre-published books whether you've read them or not b/c it just looks bad?
If you're using unpublished books as a comparison, say how you came to read them. And you might mention you also know they are not published yet.