For taking your first steps into the world of mandolin, it might help to start with a look at the best-selling method books and guides. There's the Proline Play Mandolin Today! Method Book, for instance, as well as Alfred's Teach Yourself to Play Mandolin Book. Each one of these comes with a bundled CD and DVD, so you can watch interactive lessons and play along with music clips that guide you through the learning process every step of the way.
If you're a more advanced player, you may simply be looking to expand your repertoire with new songs - books like the Hal Leonard Christmas Carols for Mandolin have that covered. Or, if you'd like to take a fresh start in a new genre that you haven't tried before, it might be worth looking at entry-level resources like the Alfred Ultimate Beginner Series Bluegrass Mandolin Basics Book and CD. A book like this will contain all the fundamentals you already know, but it will also give you some insight as to how that basic knowledge applies to a new style of music you're playing for the first time.
Of course, it goes without saying that you don't need to be the learner to shop in this section - if you're a music teacher, you're probably no stranger to stocking up on mandolin instruction materials to hand out to your students. Whether you're studying or passing on your knowledge to the next generation of musicians, this is the place to find the core resources to make it an easy and rewarding process.