"Yes, I have read the halflings are good with their slings but that was an excellent shot Pardot." Ludolf says approvingly, relaxing again as the demonic creature has been taken care of. Ludolf then starts examining the rest of the room. Investigation/Arcana:10 Also, did Ludolf identify the creature?

Those of you who were investigating the beast do recognize it as the largest quasit you have ever heard of or seen. Pardot and Klin you listen carefully trying to discern if there are others out there -- you hear nothing

The circle on the ground looks like a summoning circle to Triäsa you can tell that it has been used in the past but not anytime recently. It is a well made circle and you think very adequate for basic to intermediate rituals. 

Pardot and Ludolf as you investigate the rest of the room you are actually rather surprised to find it very very empty. There are no books at all in this room. None on the shelf, none on the ground. The rooms is depressingly devoid of literature.

Updated lower level map:

"Strange the quasit is so big.  It does not seem like the summoning circle had been used for some time," Triäsa says. She then starts to head toward the door before hesitating.

Turning back and walking up to the quasit she asks, "Did anyone check the creature to see if it might have picked up something?"  She leans in closer, speaking to herself. "Interesting, an actual fiend..." 

(Perception: 6, to check on or around the quasit for anything)

"Yes Triäsa, and fortunately not a very powerful one." Ludolf says leaning in, assisting Triäsa in her search. Investigation:7
Ludolf also ponders if the quasits blood or any part of it could be used for arcane purposes. Arcana:9 Either way he uses telekinesis, some vials from the laboratory, and his dagger to collect some abyssal blood. When finished he turns to the others. "Yes, I think we can move on now young Daventry." 

"Find anything?" As Daventry ends her look at the hallway, she draws her sword. "I think I'll offer no quarter for any more of those beasties. People entreatin' with fiends and outside forces, isn't right y'know."

Once the group is ready, she moves to the eastern door and pushes it open with her mage hand. 

None of you notice anything around the demon and Ludolf you are not aware of any specific uses for quasit blood but you know in general blood is useful, Roll nature or medicine to see how much you can collect if you so desire.

 Once again you all move back into the short hallway and carefully open the door to the east. Before is a strange room with smells of alcohol and brine permeating through it. Glass vessels large and small stand in rows on the floor and tables, ranging from one foot to six feet tall. Inside each vessel is the body of a creature floating in clear liquid. One four-foot-tall container is missing its lid and has no occupant. There are no other doors or exits from this room.

"Triäsa, could you please help me with this." Ludolf says to the young bard motioning to the vials he just brought from the laboratory.
Ludolf nature check to collect quasit blood: 17 rolled with advantage if help is given by Triäsa, otherwise go with first roll.

 "Be careful, something might have escaped captivity." Ludolfs says as they enter the room, pointing to the lidless container.
Investigation of room, particularly the contents of the glass vessels:12

Triäsa says,  "Sure, I can help."  She will assist with vials and collecting samples as he directs,  but does not hesitate to offer suggestions if she sees room to improve his methods. "What exactly are you going to use infernal blood for?  I would imagine there are a number of spells or rituals that might be aided by such a thing." 

She pauses, glancing at him a moment with curiosity. "Is this something you've done before?

"What... are these creatures?" Daventry keeps her sword at the ready, pausing a moment to look at each creature as she scans for the "missing" one.

Arcana: 6

Perception: 10

"Yes of course I have Triäsa...well, in theory at least. I admit I haven´t had much practice with all the knowledge that I devoured in my uncle´s library during all these years. It is somehow refreshing to finally put it all to use. And the blood is abysssal my dear Triäsa." Ludolf finishes smiling at Triäsa.

Ludolf fills three vials with the thick, black blood of the demon prior to moving on to the next room.

As the group of you go through the room you notice a number of creatures suspend in the murky fluid. All of them are unusual but the ones you are able to identify include a cockatrice and a giant fire beetle. There is also a jar of four severed hands all of them different and of humanoid origin. There do not appear to be creatures wondering free that might have escaped. The books in the room appear to be ledgers and notes on the creatures in the containers. All are in Fistandia's handwriting you observed upstairs in the study.

"It seems the mage was studying more than just pocket dimensions..." Pardot says quietly, clearly a bit disgusted at what he sees. Eager to move on, he gladly helps anyone looking through the ledgers and notes for another letter.

For a brief moment, as Ludolf corrects her, Triäsa's eyes flash with anger. "Abyssal!?  Well!  Fiends are of course from... um..."  The anger vanishes and she now looks flustered and flushed. "Well, I knew that.  Both refer to the evil parts of the Outer Planes."  She quickly turns, composing herself a bit as she slowly steps towards the door.  Looking back she says, "Glad I could help lend some actual experience. Though never with a creature from a different plane, I have done many animal dissections studying with my father."

She then slips out to join Daventry in her investigation of the next room. 

Ludolf had always found that correcting others would give him a sense of satisfaction, but when Triäsa leaves him, his comment about the quasits blood feels awkward and pointless.

"My uncle?" Ludolf says glancing over at Klin. "You wouldn´t know him. Suffice to say he is a sage in Luskan affiliated with the Arcana Brotherhood."

"Thank you for your assistance Triäsa. As you said, there are a number of spells and rituals that the blood could be used for, even intra-planar travel I would expect. Please accept this vial, you´ve clearly earned it." Ludolf says to Triäsa, handing her one of the vials. He then quickly turns to follow Daventry into the next room.

"Yes Pardot, Fistandia seems to have many fields of study. Why the hands though?" Ludolf says and tries to determine the purpose of keeping the severed hands in a jar.
Arcana:15

All of you looking together do not find a book with a lettered spine.

@Klin You look through the books as efficiently as you can but all you find are more and more notes on the creatures, their anatomy how they respond to different magics. You do come across a jar containing what you recognize as a Slaad Tadpode. Tapping on the glass you see it is alive and apparently very irritated with it's current situation. The ledgers and books are not arranged in any particular manner and you suspect finding any one particular piece of information would be very challenging. 

@Ludolf There doesn't seem to be a specific spell or ritual that requires the use of severed hands that you have come across in your studies. Your best guess would be either she was studying if hands of different people or races are able to produce more powerful or different types of magic or possibly she was studying how different hands respond to magic cast on them. Perhaps there is more information in the ledgers however it would take a while to find the right book.

"Be careful, that aberration may still be a youngling but it is dangerous none the less." Ludolf says scoldingly.
Arcana to determine color of slaad:9

Looking at a number of the jars, Triäsa examines a couple of them at random. "I wonder if perhaps all of these specimens might be alive.  Or just kept suspended in some way." (Nature check to see if she can tell: 18)  "The one open jar almost seems like it could have held that quasit."