Upon trying to remove the triangle, you find it is rather just an exposed piece of metal, a "doorframe" within the tree.  Feeling around, you discover the frame to be roughly 4ft at the bottom, 2ft at the top and about 5ft high.  The interior is dark to the Human eye.

How big is the base of the tree? Also what time is it roughly?

In wonder of the odd site before him, Wren steps back. "Anyone want to go tree walking?" He pulls out his lantern, lighting it and tries too look inside (without stepping in).

“Honestly, after our last experience, I’m thinking that the chances this is full of treasure and wondrous experiences is slim, while the chance this is evil and will leave permanent scars is high... screw it. Yeah, I’m right behind you.” Horus just wipes a hand across his face in semi-exasperation and steadies himself.

At the base (to support a tree this size) has a diameter of 30 feet, and it is a little before noon.

Reaching with the light, you see a circular room (about 25 ft in diameter), with a long shelf on the far wall and a table on a pedestal in the center of the room.  The walls reflect back the light, and the floor of the room is littered with old leaves and twigs.  There seems to be a long pile of them against the right wall.

Anything on the shelf?

From the doorway you can tell there is something, piles of something.  It might be leaves, but in the light and distance, you can't tell.

Cellica had been trailing after the group, keeping an eye out on their surroundings when they suddenly came to a halt. Cell peeked around to see what the fuss was and followed the others to the tree. "I agree with Horus though. Chances are this thing, whatever it is, will bite us in the ass." she mentioned dryly while cleaning the dirt from under her fingernails. "But it also might be something worth while. I don't think we should all take a peek though. If something goes wrong, just scream." In the meantime Cellica keeps an eye on their surroundings to make sure nothing creeps up on them. 

Perception: 15

Except for some songbirds, squirrels and chipmunks, the clearing and its edges are clear...

The group had been travelling for a while, and had now come to a halt in a meadow whilst examining a tree. "Perhaps it may be worth me using my detect good/evil spell before anyone investigates inside?" Seraphina offers somewhat nervously. "After all, we don't know what this place was used for, and who by"

Seraphina 

If you are casting read on

Andolin scans the room from the door.  Seeing the pile of leaves on the right, he stops. "I don't like the look of that." He nocks an arrow as Wren and Horus step inside. "Seraphina, anything that helps is welcome."

Looking at the entrance here, do I see tracks of anything that lives here?

Survival: 23 12 (Again, not sure if favored terrain applies here, additional +2)

Through the flickering torchlight (and it disturbs Darkvision), you see the room is made of the same odd metal as the frame (image 1), it is a 25 ft. diameter circle, with a domed ceiling, 8 ft high at the walls and 12 ft high in the center.  In the middle, on a metal pedestal in the center is a 3 x 3 table of the same material.  On the wall to the right is diagram (image 2) and on the left a large, concave cracked mirror.  Two 3rds of the floor, wall and ceiling are stained with a dark tint (with the shelf being the center of the unstained part). 

To the immediate right of the door, is what appears to have been a bookshelf, as there are several burned, cracked leather bindings.

The floor is littered with old leaves and other forest debris, but curiously no signs of life, not even an ant or a spider.

Already being bored out of her ass, Cellica turns around and follows her companions, peeking her head into the room and looking around quickly. She however, is a mere mortal human without any darkvision so Cellica is blind at first before her eyes adjust to the dim torchlight and she could see a little off the room and it's contents. "Well this is not creepy at all." She murmers and steps into the room (Cuz you know it's so cozy and inviting). Looking at Seraphina, Cellica nods slightly "I don't think it's hurts anyone to try and see what you see." 

"What creature would harm such a ancient force, Wildmother show them no mercy for this act." Darwin stoops his head as he follows his companions, Axe at the ready incase anything jumps at them. 

"Cant see a thing in here, its too dark." 

Should I assume you are going to investigate and give you a rundown on things?

I could try to investigate but without darkvision It's gonna be mighty shitty xD 

Investigation with disadvantage: 10 & 14 

Cellica will see if there's anything interesting, dangerous or shiny in the room. 

OCC:  The "shelf" is on the East wall, for orientation purposes.  I have put the basics (what you can see without touching) in spoilers just to break it up, unless your name is on it, then it is something your character would have noticed or know.

 Table

Mirror

 Bookshelf

 Diagram

 Shelf

 Leaves - Andolin

Horus will read through the book on fungi. Anything interesting in it?

Quote from nschrock >>

Horus will read through the book on fungi. Anything interesting in it?

 Extreme age and burns have pretty much destroyed the books, and pages tear at the merest touch.  From what you can discern it is describes different types of mushrooms, with sketches, where they can be found and what they feed on.

Horus will look through the other books one by one.