I never got into Eisenhorn books, and I quit reading Horus after like 10 books.
However, the og Space Marine book by Ian Watson is one of my favorite 40k books, it shows the daily life of SMs. Yes it is outdated but still a fun read.
Look into whatever faction you like and their books, and check novellas and audio dramas too, those are shorter and can give you a quick intro.
I also enjoyed reading Da Big Dakka, which offers a good look at Orks and Drukari.
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