Libraries are randomly generated structures, like villages and mineshafts, that occur in all Mystcraft ages.
Libraries are currently one of two methods for obtaining more Mystcraft pages. They can also be used to farm books and cobwebs.
Libraries generate anywhere In a Mystcraft Age that is touched by sunlight. They are perfectly square Greco-Roman styled cobblestone structures full of bookshelves and lecterns, with many cobwebs on the ceiling.
Though usually only one can be found near the spawn, but they also spawn in a grid like fashion every 500m, so there is no need to assume that there only is one library per dimension.
Inside of the libraries are some lecterns that will hold some pages. In the corner of the library hidden by some bookshelves there will also be a chest that holds several more pages and sometimes even holds a notebook with more pages inside.