A "secret diary" is a personal journal or book where someone writes down their private thoughts, feelings, or experiences. It is called "secret" because it is meant to be kept hidden and not shared with others. Full definition
Billie Piper starred for four seasons as the lead role of Belle de Jour in the hit SHOWTIME series SECRET DIARY OF A CALL GIRL, which delved inside the life of a legal secretary moonlighting as a high - end call girl. (sho.com)
Scent of the Past by Erin Marie Bernardo A secret diary. (iheartreading.net)
The thread that connects these two events is a tangled one, and as the death toll mounts, Madeleine must seek knowledge in odd places: behind convent walls, in secret diaries, and in the yellow stare of an aging wolf. (bookbrowse.com)