|The Travel Trilogy|
(False) life experiences of Gilderoy Lockhart
Some Hogwarts students
The Travel Trilogy was a set of three works written by Gilderoy Lockhart before 1 September 1992. Despite not being one of the required textbooks during the 1992–1993 school year, some students appeared to have bought it and took it to Defence Against the Dark Arts class, along with the other required textbooks. 
In relation to books and other media, the term "trilogy" is normally used to indicate a series with three volumes. This may indicate that The Travel Trilogy is not actually a work within its own right, but rather a compilation of three of Lockhart's prevously published works. If this were so, this might indicate why so many students had purchased it, despite it not being a required text.