Triäsa glances around at the unusual space, then kneels down a moment to touch the grass. "Is this just some sort of illusion of grass?" 

She quickly moves on to look about the space,  keeping an eye out for any more books. (Investigation: 14)

Seeing that the grass is an illusion, while the others study the crystal, Daventry heads back into the study and grabs one of the plush chairs.

"You know, we're likely wanted for questionin' in the murder of Matreous, dearly departed as a soured stuck ham. Don't mind sitting a spell and contemplating an open sky before we potentially get tossed in Candledungeon... even if this sky isn't real."

She plops the chair down infront of one of the telescopes, the one closest to the Warrior Women, and sits down, before pointing the telescope at the star that brightly shone in the other room.

Ludolf tries to pry the star map off the wall but it sparks with an arcane energy at each attempt. It is almost as if the map is part of the pocket dimension and mansion itself.  You can see and sense that there is something more to this map. You could keep trying but you get the impression this might destabilize whatever enchantment is holding it to the wall. This may lead to an unpredictable outcome. The markings on the pedestal are some sort of complex arcane spell that is far beyond your understanding.

Klin looks at the crystal and notes it is very clean as he backs away from it he trips on something and falls backwards landing awkwardly on the ground. 

Triäsa can't find any books or tomes in the room but straining against the darkness you can make out the bounds of the room find the limits of the illusion. You can make out the haze of the walls and see that  beyond the illusion is a bare, empty room. The only door is the one you came in.

Daventry sits in her borrowed chair (that thankfully does not spring to life when she goes and retrieves it from the other room). Spinning the telescope around to one of the bright stars she is nearly blinded by a brilliant intense beam of focused light that erupts from the eye piece. The radiant beam of light sails through the air and strikes the crystal which then glows faintly as the starlight remains focused on it.

"Well, so much for stargazin'." Daventry sits back in her chair.

She looks over to Klin. "Oh gods, it must be the dwarf ghost of Barleybreath, come to enact his revenge on our poor monk! Forever cursed to tumble in a drunken huff." 

Pardot looks to the telescope, and the light now beaming from it towards the crystal, and wonders if this could be done for the others. He heads a second telescope, and careful of his eyes, points it towards another one of the bright stars in the 'sky' above. If it too lights up a crystal he'll continue around to all the other telescopes doing the same.

Having exploited the room, Triäsa watches the light coming from the telescope that Daventry was looking at.  She almost says something as she sees Pardot move another. 

She comments to whoever is nearby. "If that works,  what do you suppose it will do?" She sounds rather excited to find out.

As Pardot continues with the remainder of the telescopes, Daventry goes from sit to leaning back. She responds to Triäsa. "Worst case? Explode. Better case? Drop a book from the sky. Best case? Show to us every secret of the universe." Glancing at the bard. "I'll put a silver on the second option."

Pardot goes slowly from one telescope to the next. With each telescope he points at a star another radiant beam of light descends from the illusionary sky and is focused on the crystal which glows more intensely with each beam. When all five telescopes are aligned the crystal glows to a near blinding degree and the light is suddenly redirected to the western most wall of the room. As it hits the wall the illusion along that wall drops and an archway appears revealing a hidden room off to the west. 

You can see into the room as there is no door blocking your way. What see is somehow underwhelming and yet oddly foreboding. 

This room is bare except for a bookshelf covered in chains against one wall, a plain wooden bench, and a reading desk built into the shelves. A book with the bust of a mage on its cover sits on the desk.

Triäsa gives Daventry a grin, "Not a book falling from the sky.  But even better it is a whole bookshelf!" She hurries through the arch and takes a closer look, starting with taking a closer look at what the chains are exactly.  She then moves on to the book sitting on the desk. 

(Investigation: 22)

"Wait!" Daventry, having perhaps learned something, stops Triäsa by rushing out of her chair and grabbing the excited bard's forearm. "Let's make sure there aren't any more spring-loaded specials. She packed that lock with one, don't want to find another only when it's stabbing you in the heart."

She takes a look for any traps.

Investigation or Perception: 10 +2 

"Oh? One of the locks was trapped?" Triäsa says, sounding surprised.  Looking a little embarassed as well, she sticks close to Daventry, though definitely just a bit behind.  "Maybe you can show me what you are looking for and I can help you out."  (I can either HELP Daventry or use my Investigation roll looking for traps.) 

Klin looks over the shelf not so subtly checking for traps that Daventry might have missed but nothing seems out of the ordinary.

Well that's not entirely true.

The group of you notice quickly the books on the shelves have iron covers. Each cover is attached to large, thick iron chain that is embedded into the shelf. Whatever is in these books they are not meant to be moved from this location. The chains appear to be just long enough to reach the surface of the desk. In stark contrast to the rest of the house, this entire structure is covered in dust and cobwebs. It appears these books have not been touched in a very, very long time.

Sitting on the surface of the desk is your final book. You can see the spine as it lays on the surface of the desk; the final letter is L.

Triäsa calls out loudly as she sees the 'L'.  "Yes!  It must be liberty!" She quickly calms herself and starts to take a closer look at the books themselves that are chained here. "I wonder what sort of books someone would go to all this trouble for."  She starts to look to see if there are any titles showing on the spines, or if not, take out a couple to page through. (Investigation: 18)

"Fascinating." Ludolf says as the secret room appears. "I agree young Triäsa, Liberty is likely the correct command word, very well done." He says appreciatively, before taking a closer look at the chained books. "And this should be the real treasure of this manor. Let's see what Fistandia finds worth hiding away in this elaborate way."
Investigation trying to find interesting titles, preferably associated with other planes and dimensions:9

Triäsa and Ludolf approach the chained library and nothing seems out of the ordinary. Looking at the spines of the books you actually can not tell what the titles are as the metal bindings obscure any hint to the contents. You run your hands along the books the metal is cold to the touch and as Triäsa removes one of the titles the entire bookshelf starts hum with power, a blue-purple aura surrounds the books and dark wooden structure as the whole room starts to buzz with energy. An old and powerful enchantment has been activated as the books remove themselves from their shelves still attached to their chains. They start to flail wildly in the air, angry that their rest was disturbed by a stranger. 

(I am going to assume Triäsa, Ludolf, Klin and Daventry are at the bookshelf Pardot is at the door which is only 15ft away) 

Initiative:

Daventry: 7

Klin: 19

Ludolf: 6

Pardot: 15

Triäsa: 21

Chained Library: 13

(Still last though not a one. Improvement!)

"Look out Triäsa!" Ludolf shouts as the chained library animates. If necessary Ludolf uses the disengage action to retreat back to the telescopes.
Unless concentration has been broken or 1 hour has passed since the last time books were fought Ludolf swiftly places a hex on the animated bookshelf. (Bonus action hex dex)
In case disengage action was not needed he then turns his grey piercing eyes on the construct and blasts it.
Eldritch blast:11 Force:6 Necrotic:6
"Be careful with the books." Ludolf shouts as the fighting commences.

‘Books should not be alive like this.’ Pardot thinks to himself as he readies and fires off another glowing stone towards the bookshelf.

Daventry leaps into action, having already been a bit cautioned by the rather ominous looking shelf. "Worse! That's much worse! Lud and Tri, get back!" She draws her blade, pondering a split second on how does one fight a piece of furniture.

She decides to try and chip away at where the chains attach to the shelf, perhaps wedging her blade to dislodge the connection to a flailing book without destroying the book itself. Blue light arcs down her fingers and into the blade once again.

Booming Blade Attack: 8 Damage: 13

Booming Blade on Move: 8

She prepares her shield to block a return attack.