With apologies to the late Terry Pratchett, who's ideas I'm unashamedly nicking.
Here's the basic foundation from the diskworld novels.
Books are collections of knowledge, and Knowledge = Power
Anybody who has studied high-school physics knows; Power = Energy over Time
And according to Einstein, Energy and Mass are interchangeable, and mass warps space time.
So the power contained in a book will warp spacetime as time progresses. That's just logic, right?
Ergo, any sufficiently good library will have highly warping effects on the space it occupies. To quote Pterry, "[a] good bookshop is just a genteel blackhole that knows how to read." The books go on to posit that all sufficiently dense libraries connect into one huge extradimensional 'L-Space'.
So, there's your setup. A library dimension accessible through any good enough normal library. Corridors and shelves that wind around and fold in on themselves and multiply fractaly. A non-euclidian layout with imaginary dewey-decimal numbers.
If I end up making this, it'd be a similar setup to how I did Ynn. Procedurally generating points of interest that form a linked network that goes ever deeper, and gets weirder as you get deeper. The same basic structure (location + features, roll for events every turn, navigate by 'Go Deeper' vs 'Go Back' vs 'Explore Here', depth shifting you up and down the random tables).
So, why is this interesting? Well, L-space is effectively a genteel megadungeon. Remember, Knowledge = Power, and Power Corrupts. Thus, the warping effect of the books on the world around them will produces all sorts of interesting weirdness. Monsters might be transformed librarians, characters escaped from the books written about them, animated books, things that have made their way in from other dimensions via shared L-Space. Weird features like rooms filled with candles, spacial anomalies, sealed vaults of censored materials. Spellbooks that have gotten even stranger under the altering effects of L-Space.
Why go in? The place is obviously totally packed with books, so in order that it's not a total XP-For-Gold bonanza, I'd say that only particularly noteable books are going to be worth XP. On the other hand, L-Space contains all the books you could want. Need an atlas that maps that new continent you're voyaging to? Go to L-Space. Need the historical records of that ancient evil? L-Space. Looking for a particular rare spell? L-space. Dictionaries of dead languages? L-Space.
So yeah. Not sure when I'll develop it, but the idea is there knocking about in my head.