What else is there for a true dragonborn? Read all the the other black books?

Way to spoil his name. Thanks

They are worth finding, because they have some interesting perks/skills to use, and there is an achievement to unlock if you find at least 5 of them.

And this is Skyrim... so it's pretty obvious what you should do next.
Do what ever you want. There's a lot of sidequests to find, and secrets to discover (like the battle with Karstaag)

There's also four new Shouts to find in total.

Why is this on the Dawnguard forum? Some of us don't have Dragonborn yet!