Up to your DM
This is one of those edge cases where the DM needs to adjudicate.
However, whether it should work for a book with a lock (like a modern diary, as you pointed out), or a book without such a locking mechanism, going by the examples the spell does give, I'm not sure what I would rule on this.
The spell says (PHB, p. 215):
a closed door, window, gate, chest, or other entryway ...
At least most of these are things that have locks on them, or at least can have locks on them. A book can have a lock on it (i.e. the locked diary), so then the question becomes "does arcane lock lock things that can have locks on them, but don't?"
The spell also says:
While affected by this spell, the object is more difficult to break or force open; the DC to break it or pick any locks on it increases by 10.
It says "to break it or pick any locks on it". This, to me, suggests that the objects should have a lock, since this seems to have been worded with a locked door in mind. Those are the two main ways to get past a locked door. Under this interpretation, a DM could rule that you can cast arcane lock on a book with a lock already on it, but not a book without a lock.
However, the fact that it says "or", could equally be taken to mean "if it has a lock, you can pick the lock, but if not, you'd have to break it", meaning that the door, window, etc, doesn't need to have a lock on it to be a valid target of arcane lock. But then, what stops this spell from targeting any object? Already we have an inconsistent list with "chest, or other entryway"...
So we're still left with "up to the DM". If the DM did allow it, then if someone wants to open your arcane locked book, they'd need to either pick the lock, or break it open (with great difficulty) to see the pages inside. This would, of course, be the same for any such item the DM ruled would be valid (backpacks, etc).