It consists of two parts - a database of libraries and their acquisitions information to help authors target their marketing materials to the people and regions best suited to their titles. It's subscription-based and you can find out more here.
Along with it they offer a training package to help you learn how to navigate and best use the database. Behind the Stacks, with author and 25-year librarian Judy Gann, walks you through the process and the best way to get a positive response as an author. As they say on the blog:
"The database is half the equation; the training is the other half."
It looks to be a great resource (and there's a contest for a subscription discount running until Thursday! Check the blog for more info.)
Psst: You can find them on Facebook too.