What he means is that he wishes that books on memory charms fit that description—but in fact they’re not guarded at all or even in the restricted section of the library.
That’s clearly the first level meaning. He’s wondering whether there’s a second meaning, which is a subtle hint that he has already done exactly that, maybe hoping that Harry will pick up on it and not saying it directly in case Dumbledore or someone else is listening, maybe just a private joke.
What he means is that he wishes that books on memory charms fit that description—but in fact they’re not guarded at all or even in the restricted section of the library.
That’s clearly the first level meaning. He’s wondering whether there’s a second meaning, which is a subtle hint that he has already done exactly that, maybe hoping that Harry will pick up on it and not saying it directly in case Dumbledore or someone else is listening, maybe just a private joke.