Mayor Mare,
Do you know what the trouble is with having secret passageways in your castle? They have to be built, and the only way they're kept secret is if you do it yourself, and no one stumbles across them during construction.
Or, if you're like me and somewhat naive, you hire a small crew of dedicated workers to build the passages, and expecting them to strictly uphold to your specifications without any deviations.
Gods, the foremare was so obnoxious back then, and I can't remember her name. It wasn't Hula Hoop, but I think that's close. Agh, it's right on the tip of my tongue, but she was so, SO persistent to deal with when she wasn't doing her job.
She always wanted to challenge us in some way or another, magic or otherwise. Even tried to force us to compete by "trapping" us, then demanding a competition like some kind of demented achievement hunter.
"I bet you can't eat more bananas than me!" she screamed at me through the sack opening when I tripped down a set of stairs. Ah, what a day that was. Truly, her behavior was adorable, if somewhat problematic at times.
Thinking on it, I'm certain that book was hers, probably made after the time she hosted a beauty contest to try and beat us in good looks. Only she would make something so devoted to a startlingly blunt interpretation of "beauty's in the eye of the beholder." Not even Starswirl would be so selfish, and he invented a beard-trimming spell for exceptionally long beards, for crying out loud!
Still, I would have picked a better spot to stash such a powerful book. If I had to hide it, I wouldn't have had it sitting alone in an ominous room heralding the fact that it's probably something not to be touched, which would practically offer it on a silver platter to anyone that stumbled across it and felt like touching.
Seriously, I looked at the room afterward, and there's no way I had asked for any room to have a bottomless pit. None! Why would I need one? Granted, it would be a fantastic trash chute, but the risk is too great for other non-trash things.
No, it would never be that obvious for a relic that needed three princesses to undo the damage it had caused. Why, I'd stash it in the library itself behind one of the bookshelves backed up against the wall, crammed in the small crevice between the two. Noone with any kind of sense would try to claim a book so rigorously hidden short of moving around the whole shelf or wrecking the library, and who would have the sense to do that?
Er, this is just hypothetical, mind you. I'm just thinking of a better hiding spot that doesn't require some elaborate locking mechanism and a room torn out of Sombra's Interior Design Catalog. There's certainly nothing there worth your time, Mayor. Or Twilight's. Both of you have a busy schedule to keep with those damages after this incident, and I fully expect Ponyville to be back to normal within a few weeks.
Well, "normal" meaning... you know what I mean.
Keep calm, take deep breaths, and you'll get through this week, Mayor. I'm certain nothing worse can happen to you beyond Rarity deciding your attire needing to look so business-formal it makes my sister's secretary uniform look outdated. I still can't believe she kept that.
Friends Forever,
Princess Celestia
Luna, do you remember that foremare from over a thousand years ago? The one we hired to make the secret passages?
Yeah, the one that insisted on wearing a stage cloak, and shunned hardhats.
Oh great, you don't have a name either. Well, you know who I'm talking about. Did she ever build any other rooms you know of?
Not even the underground hot spring?
Oh please, that wasn't so secret at all. That foremare had a big thing for peepholes, and that's how I found out. She really had some serious obsession problems, and--
L-Luna? Are you okay? Calm down, I'm sure they weren't used that way!
Ponyfeathers, there she goes again. Might as well prepare to explain to the astronomists why the moon has another crater in it again.