Libraries contain many bookshelves, 5 Lecterns, and as many as 5 Symbol Pages (one per lectern). In addition, there are wooden support beams in the corners, and plenty of cobblestone. If generated on a cliff or suchlike, some libraries may be malformed, with multiple "floors" among which the lecterns and bookcases are divided. Above the bookcases are cobwebs shielding a darkened ledge around the room; monsters can and likely will spawn there before the player reaches the library. Near the entrance is a secret chest containing more pages, several Sealed Notebooks, and materials such as Leather, Paper, or Ink Vials. These materials might help a stranded player to create a new Descriptive Book, and escape to an Age with more resources and perhaps even a Star Fissure.
Libraries will only spawn on the surface of the age (any place where the sky is unobstructed), so if it is a Cave World, they will be on the surface at ~Y=127. Libraries are easy to notice in Void worlds, as they stand out against the sky.
There is currently no way to disable the spawning of Libraries within an age.