Can a spellbook contain cantrips?

Is it possible for cantrips to be written in a spellbook? Their description suggests not, as well as the fact that the Wizard’s cantrips are ‘known’ rather than being in their spellbook.

For example, if a Wizard found a scroll with a cantrip on it (can cantrips even be on scrolls?), could they copy it into their spellbook and ‘know’ it henceforth?

The reason for the question isn’t that I’m hoping to know more cantrips than the rules allow, it’s for a character’s backstory I’m working on. The character will only know the allowed number of cantrips, but it doesn’t make any sense for the character to know any cantrips unless they were in a spellbook. (The character is a Rogue who stole a spellbook and started learning magic from it.)

(It’s fine if this can’t be answered now, I’m happy to wait for the DMG release if there’s no answer beyond ‘who cares, just make it up’.)