Here's advice from Rachelle Gardener, an American literary agent
In non-fiction book proposals, we always have to provide comparable titles (the "Competition" section) and increasingly, editors are asking us for comps even for fiction. Many authors write something like, "There are no books similar to mine." What it says is, "I haven't taken the time to properly research the market and I have no idea what other books could be compared to mine."
Remember, it's not bad to be able to compare your book to others people have heard of. It's good. It helps people begin to capture a vision for the type of book you've written. If you can point out the ways your book is similar and different, and why you think yours is a good complement to the other, you can further help a publisher understand what your book is all about. Don't ever claim "There are no books like mine." If that's your impression, go back to the bookstore and find some.
Good advice....and I guess A Good Confession was pitched as A London Irish Thorn Birds....the one I am working on the at the moment I hope has the absorbing action of Kate Atkinson's When Will There Be Good News combined with the rawness of Gerry Conlon's In the Name of the Father...I think...
Read more of Rachelle's advice on her blog by clicking the title of this post