From the introduction: Secrets of the Astral Library is intended for four characters with an average party level (APL) of 5. Characters who complete this adventure should earn enough experience to reach halfway to 6th-level. The characters find themselves in an extra planar library floating in the Astral Sea. This campaign takes place in Arden, although it can be set in your own world. Any party composition should be able to finish the adventure with intelligent play.