Oh, so this then was the mystery of why some characters got the tome and some didn't? I didn't know it had been solved.

That is not the reason, Al. ;)

I think the vast majority of players have their stealth skill at 100 by the time they can descend the rift and clear the library, hence it has never been a problem.
Stealth trains faster than climbing, which is mandatory to descend (unless blind but I'm not sure what exactly blindness does in this case).
It's probably something else, I had chars that had 100 stealth for a long time before getting to the library and still couldn't get the tome.

it doesn't make sense that it is a "random" factor that says you can't get it. Adom doesn't work like that.

only time i've had it denied was entering the library with sub 100 stealth.

I still have no clue how you train stealth so fast, by late game mine tends to be around 70...

Huh, I recently tried to get the tome only to discover I have insufficient stealth, went to the Rift to level up and max stealth, returned and still didn't get the weird tome on a few tries. Theory seems worth investigating in depth.

It's possible this is intentional... if you have already been a noisy nuisance, why should you get a second chance?

You already do get second chances. You can still get the weird tome if you behave noisily (e.g. clearing the library) first, chat to the Librarian, get told to "Shhh!" and then wait for an appropriate amount of time silently and perfectly stealthily.