Each diary entry refers to events in Harry Potter and the Methods of Rationality, written by Eliezer S. Yudkowsky of LessWrong.com, "a community blog devoted to refining the art of human rationality".
They are written in the style of The Very Secret Diaries, but this is just for flavor.
Each character, in some of their diary entries, displays a particular cognitive bias or heuristic. Each character has one diary entry without a day number. Across characters, entries that occur on the same day all refer to events that occur in the same chapter of Harry Potter and the Methods of Rationality, although the day number and the chapter number are not the same. Cross-referencing against the other characters, you can figure out the missing day number for each character. Index into the cognitive bias/heuristic by the day number to get a letter. Order these by the chapter numbers corresponding to the missing day numbers. This spells out FIFTH NOVEL. The fifth Harry Potter novel is the answer:HARRY POTTER AND THE ORDER OF THE PHOENIX
With (day number, chapter number), the biases and their letters are:
|T||Longbottom||Fundamental Attribution Error||(13,33)|
|H||Malfoy||Hostile media effect||(1,42)|
|L||Dumbledore||Well-travelled road effect||(11,76)|
There are also some day/chapter pairs that are not used in the final answer: