I dont know if there its an unknown use for the ostentatious bookstand that you say. But i do know that it deals drowning dmg to both monster and bosses. Check this out:
Acolyte Storm, a high lvl player and also a very active player from the mysteries community, did a really good approach on Secret Library and its mysteries. You can found the article here: Secrets of the Secret Library – Tibia Secrets
You can see him trying various of theories on the Secret Library. At one point of his article, he talks about the Magic Sphere that you asked for: A Magic Sphere spawns on top of the middle Bookstand. It can now be clicked, and is an extremely powerful weapon.
This is exactly as you described. It sends a holy beam. But it has some particularities: "Note that it skips the square directly north of it, meaning that the player who clicked the Bookstand cannot be hit by it. If we have a second player stand in its path, it does not damage the player."
It does a lot of damage, and as the picture above shows, its drown dmg. You can see the monsters lined up and beeing hit from the Ostantious Bookstand:
¿Does it hits the Boss?
Yes. Also with Drown Damage. I dont have a picture for this. But, quoting the article, we found this:
"Luckily, I have been able to hit him exactly once with this. It did about 6000 drown damage to him, and no other major effects".
I recommend you the lecture of this article as is really fun and very catching.
Source: Tibiasecrets.com; article from Acolyte Storm: Secrets of the Secret Library – Tibia Secrets