Ah, don't worry, the Primarchs themselves are the results of the work of Erda, a perpetual (seriously, what a retarded plotline the perpetuals are) that did all the heavy lifting and not the Emperor. She also had the ultimate woman moment and despite theoretical millennia of wisdom started the Warp Storm that scattered the Primarchs away because woman moment.
And then she was killed in the next book by Erebus and that's it.
No one really cares about the Black Library nowadays, it has become so retardedly convoluted that barely the hardcore fans care. If you want to read Bolter porn, you don't read the Horus Heresy for sure.