You wrote asking about the Archives. First, let me start off with this: Do not enter the Archives. It is a terribly dangerous place, and you are in by no means ready for it. You are too inexperienced and impulsive to survive more than an hour or two at most. Keep to the safer parts of the Library. The Children's Section, perhaps. Still, perhaps some descriptions of various entities and locations will sate your curiosity
The Black Dog is a creature held in the Archives. It is a thing of snapping jaws and teeth, burning eyes, and claws, of howls and growls and baying. It is all the things you fear in dogs, and nothing else. You should never encounter it, since you should never be in the Archives, and it should never leave the room in which it is held. If you do, then you have made some very poor choices, and I have little sympathy for you.
The Manticore is a very clever, very dangerous creature. Extremely knowledgeable, however. It is in the business of secrets. It will share some of its secrets in exchange for secrets of yours. Be aware, though, that it will know if you are lying. Again, ideally you will never encounter it, but it does have a desire to escape, so the possibility exists that you will find it through no fault of your own. Speak politely, speak quickly, and speak carefully. Never forget that its name means man-eater. In far-off lands, its name is boojum.
The Printer's Devil, captured in Mexico. He has well earned his damnation, and may the world burn before he leaves his cell. Carcosa is held in that fevered head, waiting to be released. If you encounter him, hold him. He is only a man, and less than the sum of his words.
Bears. Many, many bears. Several wandered in a few years ago, and they've kept on ever since. They were a nuisance for a while, but they've developed nicely, and are almost useful. The Docents take care of them, and seem rather fond. They breed rather well, but something is keeping their numbers down.
The Boilermen are generally found only in the archives, tending to the Boiler (as their name suggests). They are still fairly human in shape, though piled down with heavy leathers that may or may not be part of them. Their left hand (or sometimes their right) is replaced with a large scoop shovel which is used to move coal and raw words into the furnace. Rather than mouths, they have a metal grill which takes in the coal dust that seems to sustain them.
Leviathan is bound by silver chains in an ocean that is both smaller and greater than the one from which it was caught. I am confident you will never encounter it outside of the Archives, because if you have, it is free, and it will have shaken apart the foundations of the world. Keeping it held is one of the Library's lesser purposes.
If you find yourself in the Archives, there are a few safe places. One of these is Xanadu. The stately pleasure dome exists within the Library. I do not know if it owes its existence to Coleridge, or if Coleridge was somehow inspired by the Archival version, but it matches his description closely. If you make it there, you should be safe until a Docent can find you. Watch out for flashing eyes, however.
If you should somehow wander into a genre, do not be alarmed. It is a pantomime. The 'people' are stock shadows. Cowboys in western towns, private detectives going through the motions in noir mires, and gaudy heroines in mawkish romances. Ignore them. Why these useless pastiches exist, I could not even begin to guess. It's possible that someone created them, though someone wanting such a spectacle boggles even my imagination.
There is, somewhere in the farthest reaches of the Archives, an old dull-skinned serpent. Not that serpent. Nor, on the other hand, the one you might have wanted it to be. This serpent will poison the sky and kill the son of the gods when the world ends, after dying itself. In the meantime, it waits in the Archives. It is not a prisoner, insofar as it does not care to leave before time. It does not know that its ending has been unwritten, and for its own sake, we shall not tell it. It enjoys reading French poetry.
The Librarians have their own quarters somewhere in the Archives. The Docents and Pages, at least; the Archivists do not, so far as I am aware, sleep. I have never seen these chambers, however, and know of them purely from speaking with Pages. They are either well-hidden, or else purely inaccessible. Occasionally, you might find a parcel of them moving onwards towards work. Only the more senior of their number are encountered singly, and only in places where they feel safe. I have no idea what they're on guard against, but it should be yet another reason not to go wandering the Archives.
You will of course have noticed the clock above the Archivists' desk in the Library proper. It is run by a great quantity of clockwork. There is a great chamber full of giant chains and gears and cogs and whatever else you call that collection of mechanical clutter. It's terribly easy to lose an arm in there, or find yourself crushed. While it all moves, no one has ever heard the bell at the top chime. I once climbed up and attempted to strike the bell physically. However, I found I could make no sound no matter hard I swung my hammer. I could have made more sound hitting a mattress.
The Angel Jerahmeel is trapped inside a cage of starstuff, the beard of a child, and the sound of lost hope. She does not speak, but will write if you give him the materials. However, be aware that she is capable of lying if he thinks that it's necessary to do so. Do not trust her any further than you have to. He has motivations beyond mere escape.
I suppose by this point, we can throw away the pretense. If you're reading this far, you've disregarded my advice, and you're in the Archives. It's not difficult to get there, after all. Getting out is more difficult.
From where you are, go to the card catalog. Any Librarians in the place should be able to show you where it is. You will know you are in the right place from the vast quantity of filing cabinets. You will have a sense that you are being watched. This is likely from the Archivists' field of vision. They use this room to track the books under their care.
From here, you'll want to go through the green door. This will lead to the origami room. Be careful of papercuts. Everything, from the trees, to the furniture, to the birds flying around is made of folded paper. If you unfold anything, you'll find something or other written on it. The furniture tends to show plans, while the birds are sheet music, except for the eagle-owl, which is Cardenio. Don't spend too long. You never know what might be watching you.
The next room will be filled with candles. Watch for dripping wax and shade your eyes. The room is just a hair above pleasantly warm. How the smoke makes its way out is beyond me, but unimportant. Carefully so as not to ignite yourself, make your way to the red door with the number three carved into it. There is another red door in the room, but it does not have a number. Make sure you are going through the correct one. You do not want to get lost. There are, as I have stressed repeatedly, dangers in the Archives.
Now, you'll find yourself surrounded by insects. Wasps, specifically. Don't worry. Their stings are gone, replaced with nubs. If one lands on you, it might write something down. Take no mind. Their warnings are stuff and nonsense, and their poetry is doggerel. Just keep following my directions. Dig around the paper on the floor until you find a manhole. Descend. Quickly, you haven't much time.
Now you will be a sewer. As the smell will doubtless inform you, it runs with acetone, not the more… organic waste you might have expected. Do not fall in, unless you are particularly fond of chemical burns.
You can hear it now, can't you? The baying. The growls, just a room or so behind you. I could pretend I didn't know it would turn out this way. But we're past the time for lies. I'm sorry. I want to live too.
But I can give you a chance. I only promised to bring you to it. I never promised it would catch you.
At this point, you're probably in the ink room. Take the pen from the pot. Pour it out. There, there's a door. Ignore the garden, open the door. Push past the birds. Ignore them, and say the name of your father, and the door is open. Keep moving. Ignore the breath on your neck. Yes, you're now on a cloud, but keep moving. New door is open, keep pushing through. Don't stop to play poker. He can't help you, and doesn't intend to. Next room. You're going to want to stop to clean. That is an effect of the table, ignore it and push on.
Now you're in a room with concrete walls and broken pieces of rebar. Books impaled on them. Be careful not to injure yourself. Look at the bloody prints on the floor. I'm sorry. I lied. You won't get out of this.
I want to live too.