Boundless Athenaeum

You use chalk to inscribe arcana sigils upon a wall or vertical surface. At the completion of the casting, a 10 ft. x 10 ft. portal opens up within the space of the sigils, leading into an extradimensional library unique to the caster. Any objects created by this spell dissipate into smoke if removed from the library, and when the spell ends, any creatures inside the extradimensional space are expelled into the open spaces nearest to the entrance, as well as any objects other than books or other reading materials. While this portal remains open, the library and everything within it appears to be on the same plane of existence as the portal. 

The library is well lit, and always at a comfortable temperature. Decor is chosen by the caster at the time of casting, and can include desks, writing tables, and shelves. The library is never smaller than a 10 ft. x 10 ft. room, but will grow to accommodate any number of books added to it.