so i have been playing the warhammer 40k games on my pc. and i was wondering what is the first book id want to read for this series?
I would recommend you The Beast Arises series (Space Marines campaign across universe; 12 books), Eisenhorn trilogy (Inquisitor adventures; 3 books), Cadia Stands & Cadian Honour (Cadian/Eye of Terror war; 2 books) and Gaunt's Ghosts (Commissar Ibram Gaunt adventures; 22 books). You don't need to read all, just pick or taste a theme you like!
Alternatively, you can also read Dawn of War (PC game novels; 3 books).