Screenwriting : Requesting rights information for adaptations by Rachel Metzger

Does anyone have advice / best practices for requesting information about screenplay rights to a book? I've tracked down the rights holder to a book published in 1970 that I'm interested in. As far as I can tell (at least from IMDb Pro) no one has this project in development so I'm hoping the rights are available (and cheap!). I know that my next step is to write to the rights holder, an agency in NYC and, well, ASK... but before I do so, I wanted to check here and see if anyone has any advice before I go off exposing myself as the completely inexperienced dweeb that I am. Any advice on how to word my request? Any similar experiences to share? Sample rights query letters? A couple of Xanax? Thanks!