These books are broken up into two parts Historical and Contemporary.
Historical meaning books that are focused on History, Archeology things of this nature. A lot of Recon Druids focus on these books.
Contemporary meaning books focused on modern understanding and also related to some of the Orders. These books have things mostly relating to the Neo-Druid.
These books will help further your studies in Bardic, Ovatic, and Druidic paths. These will be grouped into categories for easier understanding.