So what are those books about, just d0rfs killing endless hordes of örks?
Yes. He also wrote another book called "die Orks" which was as cheesy, but quite funny nevertheless because it was about a troop of orcs that killed endless hordes of humans.
What? No, Die Orks was originally an English book by an English-speaking author that became a huge hit over here when it was translated to German, and Heitz's publisher used his Die Zwerge as a cash-grab riding on the wave of success of Die Orks.
Basically, Die Zwerge is a typical "Oh hey, look at that book selling like hotcakes? WE CAN DO THAT TOO!" kind of me-too story.
EDIT:
Just checked wikipedia and yeah, that's basically the story, only it's even more blatant: the publisher Heyne actually called for authors to write "The [fantasy race]" knockoffs when they saw the success of the German translation of the English book trilogy "Orcs: First Blood", which was simply titled "Die Orks" in German.
This spawned "Die Zwerge", as well as "Die Elfen" and "Die Trolle". The only common fantasy race they left out was, apparently, the humans.