I dunno the series and the picture quality isn’t great, but it looks like there are duplicates of the same book several times in there. As far as I could see from the wiki page, there are 64 books listed and only one (The End of Death) has the same title for a 3 volume series. Yet in the picture there are multiple copies of First Heretic, Outcast Dead, Tales of Heresy, etc.

https://en.wikipedia.org/wiki/The_Horus_Heresy?useskin=vector#Titles