In the catalog Ninth of Av is a day of mourning commemorating several tragedies that happened to occur on this date. Traditionally, one would fast from sundown to sundown, recite lamentations (kinot), and read the Biblical book of Lamentations.
Books on Tisha be'Av: BM 695 T5.7
Kinot and Tisha be'Av liturgy: BM 675 K5
Lamentations: BS 1531-1535.5
This site gives an overview of the history and practices of this day of mourning.
Introductory article on the origins and customs of the day.
This site contains articles on the basics of the day as well as the traditions, fasting, and the Temple and Destruction.