referral
I came cross this website by accident when researching Sherman & Associates, after choosing this agency to query (re: my first novel) based on a professional referral. I was impressed with it's list of clients, and felt we were a good match, if they feel they can market my work, of course. One of their newest client's first publication, "Souvenir," was great and somewhat akin to my book, giving me more encouragement. (I think the author posted here earlier.)
Anyway, it was good to read so many positive comments about the agents and their professionalism.
Good luck to all, whether querying or selling your book.