The Disgaea novels can be found in bookstores, online retailers like Amazon, or at your local library. You can also check with publishers or specialized bookstores that carry Japanese light novels or manga.
That has not been confirmed yet as of today. However, there is something called the Disgaea Novels that supposedly reveals more info about Laharl, Etna, Flonne and the rest of disgaea characters. The novel takes place 10 years later, revealing new characters and relatives of Flonne and Laharl. So look for Disgaea Novels, you may find your answer in there. Also, check out Disgaea 4 it may reveal some updates about Laharl, Etna and Flonne.
The game "Disgaea: Afternoon of Darkness" played on PSP has been adapted into a manga called Disgaea or in romanji Makai Senki Disgaea. There is also a Disgaea 2. So, yes, it is a manga.
Disgaea Infinite happened in 2009.
Mugen Keitai Disgaea was created in 2004.
so far there are 12, im not certan of disgaea 2 and 3, but the anime disgaea with laharl, there are only 12.
Disgaea Infinite was created on 2009-11-01.
The Disgaea anime is just one season with 12 episodes.
Yes, there is one based on the game. it's called Makai Senki Disgaea.
So far, there is only one: Disgaea DS
My opinion is "Disgaea Afternoon Of Darkness" so far but i am trying to get to play all of the games.
In disgaea 1 you can teach the cleric offensive magic by getting a mage/skull pupil. You can also use team attacks. In disgaea 2 the cleric you can also use tower attacks.
Disgaea 3 can be somewhat hard to find. If you are in the US, some GameStop stores still carry D3. Your best bet might be to check online retailers. Nippon Ichi's own website would be a good start.