Assassinorim, kingmaker. 3 members of the ordis try to depose a ruler of a knight world and set up a more pro imperium one.
If he likes that recommend him the Infinite and the Divine. It is a necron book but it's by the same author and same writing style as the above book but it's overall stronger and I agree a little with recent sentiment that it's probably the most newbie friendly 40k book out, even over the old staples like Cain, Guant, or Eisenhorn.
For character stuff I actually think some of the short stories are interesting. The last church often gets pushed as the best short story but I particularly like two of the malcador focused ones (Sigilite and First Lord) and a recent one was one that depicted Kahn's and Angrons first meeting after the latter literally got abducted from the middle of his rebellions last stand, although I'm sure other users have some noteworthy shorts they can name.