Dungeons & Dragons DND Armoire-Library

A Disguised Portal:

This armoire-library is not what it seems. Upon examining a particular book—perhaps a dusty, untitled volume, or one bearing an arcane symbol on its cover—the characters might activate a magical portal hidden behind the shelves. This portal could lead anywhere: a secret chamber, another dimension, or even the villain's lair.
Perhaps the armoire itself is a magical object. By uttering a key phrase or touching a specific point, it reveals itself as a portal.

A Repository of Forbidden Knowledge:

The books in this library might contain crucial plot information: a monster's weakness, the location of a lost artifact, or the secret history of a place. However, some of these books might be protected by magical traps, riddles, or even spectral guardians.
Certain tomes might contain dark or forbidden knowledge, the reading of which could have negative consequences for the characters (loss of sanity points, temporary curses, etc.).

A Secret Hiding Place:

The armoire could have a secret compartment, perhaps behind a false row of books or in a hidden drawer at the bottom. This hiding place could contain treasures, important documents, or even a hidden NPC.
An ingenious mechanism might activate this secret compartment, requiring a Perception and Investigation check to discover it and a Dexterity check with thieves' tools to open it.

A Decor Element with History:

Even if it hides no secrets, the armoire-library can tell a story about the place where it is located. The types of books, their state of preservation, and any notes or markings on them could provide clues about the former occupants of the place or the events that happened there.
Perhaps a particular book has a dedication to an important character in the campaign's history, or contains handwritten annotations that reveal additional information.

An Environmental Challenge:

In a ruined or abandoned setting, the armoire might be on the verge of collapse, presenting a danger to characters attempting to interact with it.
The books could be infested with tiny creatures (such as magical insects or even small paper elementals) that attack anyone who touches them.

Additional Ideas:

A cursed book: A tome that, once opened, casts a curse upon the reader.
An animated book: A book that transforms into a minor monster to protect its contents.
A teleportation book: Upon reading certain words, the character is transported to another place.
A book of illusions: A book that displays illusory images and sounds when opened.