My American colleagues at Random House U.S. have just created this marvellous catalogue of graphic novels and manga published by the various imprints we represent (Pantheon, DC Comics, Vertical, Titan Books, Villard and Del Rey). All the different series are highlighted and described, (and at the back of the catalogue all the volumes are listed), plus they are organized into YA and Adult categories. Any adult ones that are suitable for mature YA readers are starred. There are also some great collection development tips provided by Robin Brenner, a reference and teen librarian at the Brookline Public Library, and editor of the teen graphic novel review site No Flying No Tights.
You can access this catalogue online here (just scroll down to the bottom of the page) and either browse the PDF online or print it out. There's some great stuff here for public and school libraries alike.