Some of the libraries have books, some don't, and others use the SL environment in new innovative ways.   Add your info on new ones!

Alliance Library (Mature--some mature materials in the library)

A nicely diverse collection that includes:  general reference, digital collections, human sexuality, second life info, literature, sports, westerns, arts and crafts, cooking, anime, and Manga.  It also has a large collection of SL serials, newspapers, etc. on the main floor.  Definitely worth visiting.

http://slurl.com/secondlife/Info Island/111/69/33/

_black library (Mature--seems PG)

A great collection of Fiction, Non-Fiction, Art, Music, and Games.  The "books" are set out in the library, and clicking on one will give you a notecard with the story.  Also the location of fairly regular meetings on teaching in SL.

Associated group is: Blackboreders

http://slurl.com/secondlife/Hyperborea/92/59/24

 Info Island International (Mature--seems PG)

There are a number of libraries to explore on this island, many of them from universities.  At the main site, you also find links to a number of resources, ranging from Google to encyclopedias.  Very much worth exploring the area.   Reference librarians keep regular posted hours at the desk.

http://slurl.com/secondlife/Info Island International/115/246/33