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Recommendations for Warhammer 40k
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As someone who also just started out with Warhammer 40k (and also got onboard a few friends with this lol) I’d say the Eisenhorn trilogy is a great place to start.
It's basically a space thriller. And while extended lore is present, it's not required to enjoy the story. Whathever is mentioned gets explained and makes the whole trilogy very beginner-friendly.