Sometimes query letters have references to other works. According to a literary agent’s assistant at http://brainbombs.blogspot.com/ (blog is now defunct), about half of the queries that they’d selected for manuscript request this week have a line that says something like, based on your representation of such-and-such book OR interest in such-and-such genre, I thought you may be interested in my book XYZ.
She’d also seen the tactic where the author says, My book XYZ is similar in the themes of love and forgiveness in such-and-such book.
After a discussion with their in-house editor this is what she had to say: Mentioning a the work of an agent’s client is 1)flattering and 2)let’s them know you’ve put some thought into selecting them as an agent to query. Comparing your book in tone or theme or culture to another book gives the agency some sort of idea on your writing style.