I think I read about them, too, but not as a different Item. You rather take a normal spellbook and use it as travel spellbook. The Big Tome with all your spells (or the collection of spellbooks) stays in your tower, you only take your travelling spellbook (s) with you: it contains the spells you use when you travel, not the ones you use in magic research, spying and scrying, and item creation. Or it is just another set of your spellbooks, protected (resistance to fire, waterproof, such things), which won't bereave you of all your spells when they get destroyed (after all, you have another set of spellbooks at home)