I wrote a super long response and deleted it.
The gist was the book was decent insofar as I read it because I wanted to read about World Eaters tearing Emperor's Children apart.
As far as this story being about Kharn, it wasn't really. It was more about the final moments of the World Eaters as a unified legion. They really dropped the ball on the Kharn stuff, imo.
Are we supposed to assume that a sequel will come out? I never got that impression from BL about anything, other than the main storyline of the HH.
Maybe not everything in the 40k fluff needs to be fleshed out? Angron and the butcher's nails added a surprising amount of depth to the World Eaters, but in 40k they're pretty one dimensional. As far as Kharn himself, I see him sort of like Wolverine and Batman. A mysterious past is pretty cool, but when you try to show the audience exactly what happened in the past, it sort of dilutes the cool mystique of the present (or future, as the case may be).
Just my 2 skulls worth.