Where the first Disgaea manga just about entertained, the second book, which bridges the gap between the games, is a far better read.
It starts with Etna quitting as Prince Laharl's Vassel and attempting to secure herself a new title so that she can start her own empire. The story soon moves on to a new scenario, whereby a young man named Alex, the last non-demon inhabitant of his world meets Rosa, a spoiled demon princess whose gotten sucked out of her home by a spell gone awry. The two become bound by a contract, and very shortly the quest is on to get Rosa home.
To start with, the artwork is far less muddy, and the panelling flows far better. Even though it's still jumbled into a sort of stylised mess, the artist actually pulls it off very nicely. There's some gratuitous nudity here and there, but it's Disgaea and therefore naughtiness should be expected.
The book also does a far better job of conveying the "worldview" of Disgaea. There are some wonderful character cameos, all performing various everyday jobs. Following Etna's quest to gain a new title is a fantastic way to show the way the character grading system works, and all the characters are so damn cute it's easy to forget they're all evil demons. This is definitely evil with a kawaii face!
The book also has a bevvy of extras - there are character profiles, some fun "next volume" skits and a very thorough set of translation notes.
The only problem is that the books are becoming increasingly hard to find. That said, they are worth tracking down, just don't expect to pay list price for them.