Minecraft wiki entry for Stronghold, associated text: There are exactly 233 bookshelves in a large library, yielding 699 books if mined without Silk Touch, which is 10 stacks plus 59 left over.
Although strongholds do not generate above ground, they can be exposed through above-ground dungeons or canyons.
Sometimes, they can also be exposed underwater if they generate in terrain that is close to an ocean, or exposed by an underwater canyon.
When generating in a buffet world, many rooms lack the outer wall layer, causing the stronghold to appear fragmented, with the exception of the portal room.