A tome is a large book. If you're pre-med, chances are you're going to have one heck of a tome for your biology class. Tome is often used to refer to a book that is not only really large but also unusually important.
If you generally carry more than one tome to class, invest in a bag with wheels so you don't hurt your back. A tome is big and heavy. "Wow! This tome weighs a ton!" Whatever you do, don't leave your tome at home—you'll wind up in detention! You can also use tome sarcastically to describe a book that clearly isn't large or important. But sarcasm is for advanced users only.