Why is this guy so under-developed in the HH series? Every other Primarch has had their own book.
He very well may get one.
Primarily, though, it's because it's pretty difficult to flesh him out in such a way that hasn't been done with other characters. Grim, brutal stoicism and total disregard for losses is already a hallmark of many other Primarchs, so it'll be difficult to translate that to Mortarion in an original way.
>every other primarch has a book has their own book.
Which book is for Sanguinius?
Did you forget about Flight of the Eisenstein? Also, a book is in the works about the fall of Mortarion and his legion to Nurgle.
'Deathguard' by James Swallow, I believe.
Sadness, melancholy, regret, a crushed activism turned to bitter pessimistic passivity. Most of it is a novel recombination of other Primarch traits, which is par for the course. Perhaps an unusual conception of purity.