I actually have an anecdote from a friend on the spellbook, he was in a game where magic was illegal.

Something happened that caused the town guard to search his house, since wizard this is bad. But all was not lost, he chucked his spellbook in the Fireplace and evaded the fuzz.

Because he was a scribe wizard, this was a mild inconvenience.

Stealing the book is a one combat problem, and that wouldn't be worse than a focus or component being stolen, every caster needs to worry about that.