I have been an avid Warhammer 40k fan for about 15 years. I was initially lured in by Dawn of War (PC Game) and the incredibly detailed lore. Having read an embarrassing amount of Warhammer books, I can say there are some real SciFi gems, a lot of good filler if you like the universe, and a few really awful books.
If you are looking to get started I would recommend the Eisenhorn trilogy, The Original Blood Angels Omnibus, or The Emperor's Gift. Dan Abnett, Aaron Dembski-Bowden, and Graham McNeil produce consistently enjoyable 40k books.
The Eisenhorn trilogy, and its follow-up, the Ravenor trilogy, are superb. I re-read both over the course of the pandemic and they both hold up incredibly well.
The solar war books are getting quite good. The latest one saturnine is actually a compelling story. Same with some of the watchers of the throne series. Also being a technologist I always like bellisarius cawl so really liked the great work.
If you are looking to get started I would recommend the Eisenhorn trilogy, The Original Blood Angels Omnibus, or The Emperor's Gift. Dan Abnett, Aaron Dembski-Bowden, and Graham McNeil produce consistently enjoyable 40k books.