The Discworld Diaries are a series of themed diaries

Diary

A diary is a record with discrete entries arranged by date reporting on what has happened over the course of a day or other period. A personal diary may include a person's experiences, and/or thoughts or feelings, including comment on current events outside the writer's direct experience. Someone...

 based on the Discworld

Discworld

Discworld is a comic fantasy book series by English author Sir Terry Pratchett, set on the Discworld, a flat world balanced on the backs of four elephants which, in turn, stand on the back of a giant turtle, Great A'Tuin. The books frequently parody, or at least take inspiration from, J. R. R....

novels by Terry Pratchett

Terry Pratchett

Sir Terence David John "Terry" Pratchett, OBE is an English novelist, known for his frequently comical work in the fantasy genre. He is best known for his popular and long-running Discworld series of comic fantasy novels...

. Each one (except the 2008 diary) is based on an Ankh-Morpork

Ankh-Morpork

Ankh-Morpork is a fictional city-state which prominently features in Terry Pratchett's Discworld series of fantasy novels. As cities go, it is on the far side of corrupt and polluted, and is subject to outbreaks of comedic violence and brouhaha on a fairly regular basis...

 institution, and has an opening section containing information about that institution written by Pratchett and Stephen Briggs

Stephen Briggs

Stephen Briggs is a British writer of subsidiary works and merchandise surrounding Terry Pratchett's comic fantasy Discworld. The Streets of Ankh-Morpork, the first Discworld map, was co-designed by Briggs and Pratchett and painted by Stephen Player in 1993...

.

The diaries feature a great deal of background information, far more than could reasonably be put into the novels. However, some of this occasionally finds its way into the series proper - the concept of female assassins and the character of Miss Alice Band were two notable ideas that first appeared in the Assassins' Guild Yearbook.

All the diaries are illustrated by Paul Kidby

Paul Kidby

Paul Kidby is an English artist. He was born in Northolt and is currently living and working in Fordingbridge, New Forest. Many people know him best for his art based on Terry Pratchett's Discworld, which has been included as the sleeve covers since Josh Kirby died in 2001.He drew a lot during...

.

The diaries are:

• Discworld's Unseen University Diary: 1998; the cover art features the character Death
  Death (Discworld)
  Death is a fictional character in Terry Pratchett's Discworld series and a parody of several other personifications of death. Like most Grim Reapers, he is a black-robed skeleton usually carrying a scythe...
  
  , possibly the character who appeared in the greatest number of Discworld novels.
• Discworld's Ankh-Morpork City Watch Diary 1999: 1999; the cover art features the character Commander Samuel Vimes
  Samuel Vimes
  Samuel "Sam" Vimes is a fictional policeman from Terry Pratchett's Discworld series. As of his latest promotion, his full name and title is; "'His Grace, His Excellency, The Duke of Ankh; Commander Sir Samuel Vimes": When serving as Ambassador for Ankh-Morpork, he is also referred to simply as...
  
   of the Watch
  Ankh-Morpork City Watch
  The Ankh-Morpork City Watch is a fictional police force within the Discworld series of books by Terry Pratchett. It is based in the city-state of Ankh-Morpork on the Discworld. The Watch was originally two units, the Day Watch and the Night Watch which were combined after the events of Men at Arms...
  
  , His Grace the Duke of Ankh, in his beloved street uniform, in other words, battered Watchman armor.
• Discworld Assassins' Guild (Yearbook and) Diary 2000: 2000; the cover art features the character Lord Downey, the Assassins' Guild
  Ankh-Morpork Assassins' Guild
  The Ankh-Morpork Assassins' Guild is a fictional school for professional killers in Terry Pratchett's long-running Discworld series of fantasy novels...
  
   leader, with his specialty peppermint (rumored poisoned).
• Discworld Fools' Guild (Yearbook and) Diary 2001: 2001; the cover art features Dr Whiteface, the Fools' Guild leader, bursting through a paper hoop.
• Discworld Thieves' Guild (Yearbook and) Diary 2002: 2002; the cover art features a "photofit" of Mr Boggis, the Thieves' Guild leader.
• Discworld (Reformed) Vampyre's Diary 2003: 2003; the cover art features Mr John Not-A-Vampire-At-All Smith, head of the Ankh-Morpork Mission of the Black Ribboners with a cup of steaming brown liquid, likely coffee or hot cocoa.
• Discworld Post Office Diary 2007: 2007; the cover art features Moist von Lipwig, the Postmaster of the Ankh-Morpork Post Office, wearing his token golden suit and wingèd hat, with Lipwigzers on either side of him.
• Lu Tze's Yearbook of Enlightenment 2008: 2008; the cover art features Lu-Tze in the lotus position, with his broom in front of him, against a square with the phrase "It won't get better if you pick it" (from the Way of Mrs Cosmopolite). It is officially spelled Lu Tse's Yearbook of Enlightenment.

There were no diaries published between 2004 and 2006; The Discworld Almanak

The Discworld Almanak

The Discworld Almanak is a spin-off book from Terry Pratchett's Discworld novels, in a similar format to the Diaries and Nanny Ogg's Cookbook...

by Pratchett and Bernard Pearson

Bernard Pearson

Bernard Pearson is an artist best known for his sculptures of Discworld characters and buildings. He initially produced figurines at Clarecraft, which he co-founded, and upon leaving began his series of highly detailed Discworld buildings, most notably a multiple-piece Unseen University.He...

was published in 2004.

Due to their limited edition nature, Discworld Diaries become increasingly valuable as they grow older. The 1998 Unseen University Diary currently retails at around £110.