Hi twotone.
If you are in the room where Elyssa was, there is a passage leaving this room behind the altar with the decomposing skin on it. There is a lever on the wall in the passage to open the gate. This passageway leads you out in the vicinity of the library. Hopefully you can look at your map and find the main entrance from there.
I hope that helped you somewhat!
BLKCat (Not the best at verbal mapping!)
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...Events are cowards: they don't occur singly, but instead they run in packs and leap out at you all at once.
(Excerpt from "Neverwhere", by Neil Gaiman)