I share the general sentiments of the sheer amount of variety one could encounter while wandering around.
For me, I always envisioned the Library (which, in headcanon, is always populated by a lot of patrons at all times of the day) as sort of amalgamation of all different architectural styles and environments mashed together, sometimes at random.
The central hall follows very closely to Blue's picture: in the vein of libraries constructed in the mid-Victorian era, with grand, soaring ceilings and solid, heavy shelves made of wood lined with impeccable plush carpeting, of varied colors depending on location.
Outside the central hall, however, everything is fair game. Rooms start off wide, general in nature, and then as you start to venture further and further out, they get more specific and specific in terms of stories told. Past the conventional libraries of all varieties and all the vast seas of submerged shelves, you'll find yourself walking from a shaft-like cylindrical room dedicated to some conflict fought between 'chipped' and 'original' automatons who gained sentience in 2198 into a dimly lit recreation of Charles M. Shultz's studio, circa February 12, 2000. These rooms, located on the outermost and oftentimes remote areas of the Library, might not be ever appreciated, or even visited, but they exist for the sole reason that they do tell a story, no matter how niche or obscure it is.
Of course, that raises the question: if someone wanted to find a item or room in particular, how are they going reach the location?
To that end, I sometimes find myself daydreaming of the wide variety of transportation methods that would be available, which would comprise, like the aesthetics of the library, of all different levels of technology and origin. Elevators with built in search-system consoles, able to get you around to nearly anywhere. Biplanes parked on makeshift runways between shelves, sharing space with winged creatures and their handlers. A 1979 Ford Fairmont rattling down a highway slicing across high-rising metallic shelves housing electronic data while a monster mechanized worm carrying a bunch of patrons whips by in the other lane. Sprawling railroad systems, employing all means of motive power and rolling stock built by many different worlds. Submarines or other sorts of pressurized vessels for patrons who cannot survive underwater, equipped with manipulator devices to retrieve books. The possibilities of getting around are endless.
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