The Disgaea novel series is an expanded universe of the Disgaea game series. It starts with an adaptation of the first game and continues with its own thing, and adaptations of the sequels were added to the universe as the games got released.

There are currently 17 novels in the series:
Disgaea: Enter the Maoh - adaption of the 1st game.
Disgaea: Revelations - beginning of expanded universe content; introduces Laharl’s aunt, uncle & cousins.

Disgaea: Returned - Lamington invites Laharl to Celestia. The novel introduces Flonnes parents and her sister. Jennifer’s villainous father returns. An online translation can be found here.
Disgaea: On Love (Part 1) - Laharl & Flonne get stuck time traveling and meets his mother in the past. Meanwhile, forces in the Netherworld conspire against Laharl.
Disgaea: On Love (Part 2) - the conclusion of the On Love storyline.
Disgaea: Battle of the Maohs - crossover with Makai Kingdom.
Disgaea 2: Mask of the Maoh (Part 1) - first part of the adaption of the 2nd game.
Disgaea 2: Mask of the Maoh (Part 2) - second part of the adaption of the 2nd game.
Disgaea: Heart of the Maoh - Laharl gets turned into a baby; Etna & Flonne travel through time to find a cure.
Disgaea: School of the Maoh - a parody of High school themed anime with characters from Disgaea, Disgaea 2 and Makai Kingdom.
Disgaea 3: Brave and the Maoh (Part 1) - first part of the adaption of the 3rd game.
Disgaea 3: Brave and the Maoh (Part 2) - second part of the adaption of the 3rd game.
Disgaea: Three Dash! - short stories with Disgaea, Disgaea 3 and Makai Kingdom characters.
Disgaea: War of Prinny - a loose adaption of Prinny: CIBTH.
Disgaea 4: Power of Iwashi - adaption of the 4th game.
Disgaea: Maoh of the Dead - the Disgaea trio makes a zombie movie.
Disgaea D2: Revisit - adaption of the direct sequel to Disgaea Hour Of Darkness..