I haven’t really been reading that much these past couple of years, but after getting into Warhammer 40k last year I’ve actually been reading a lot recently. Besides the faction specific lore in codexes, I have also been reading four Novels set in that universe in the past year.
The 40k universe is of course just absolutely massive, but I think it is much more accessible than you think. I got my start by watching a couple of YouTube videos and just stuck around. The only things you need to know are that human super-soldiers exist, half of them turned traitor 10,000 years ago after being corrupted by demonic powers from a parallel universe called The Warp, the galaxy spanning empire they were part of has turned into a fascist, religious state that is slowly falling apart, and there are a bunch of alien races who are making things even worse.
The first book I read was The Infinite and the Divine by Robert Rath, which is actually largely an odd-couple comedy. The book is about two Necrons, part of an ancient alien race who sold their souls for immortal metal bodies millions of years ago and mostly have been in statis sleep since a devastating war around that time. The two are fighting over a McGuffin they don’t really know the true potential of. The book is hilarious and very well written and I think a really good introduction to the setting in general. The only criticism I have is that the big fight in the finale is a bit too long and feels stretched for no real gain, but otherwise the book is very good.
The second book I read is Warboss by Mike Brooks. Its about Orks and a leadership struggle while assaulting a gigantic human city. Orks in 40k are giant mushroom monsters who are part football hooligans, part Mad Max ramshackle venturers. They are the primary comedic relief race in the setting. The book was pretty decent and the scene-to-scene writing but unfortuantely the overall plot feels not that well developed and shortened as you have to juggle multiple sub-factions of orks in the span of about 300 pages.
In contrast, the second Mike Brooks book I read was just excellent. It is called The Lion: Son of the Forest and deals with Lion El’Johnson who is one of twenty primarchs, in essence genetic blue-prints and generals of various Space Marine subfactions. During the Horus Heresy, the event where half the Space Marines turned traitor I mentioned earlier, the Legion he led fell into a civil war. Lion himself was grievously wounded and put into a stasis sleep. Ten thousand years later he awakens and attempts to piece things back together with humanity under siege and the loyalist contingent of his genetic sons still hunting down stragglers they believe to have been traitors before. The book is just fantastic with great action scenes and some very interesting character writing for both the Lion himself as well as the various members of his faction he encounters.
The book I am currently reading is on the other side of this conflict and deals with the situation of a chaos legion. Soul Hunter by Aaron Dembski-Bowden is about a group of Night Lords Space Marines who turned traitor in the Horus Heresy basically because they were always the ones for the dirty work to begin with, primarily using terror tactics and fear as a weapon. They are actually mostly opposed to the demonic entities that caused the Horus Heresy, but have to deal with the creeping corruption in their ranks too, as well as just the sheer attrition of men and material over the past 10,000 years. At the same time they have shoddy leadership and are bullied into being used as cannon fodder by a larger group of Chaos Space Marines. The book again has excellent character writing and inter-personal struggles and gives a pretty neat perspective on what makes these people tick.
Overall I am very impressed with the surprisingly high quality of all of these books, particularly compared with some other brands of games (cough Magic cough) and their shoddy lore output. The version of Soul Hunter I have is actually part of an Omnibus collecting three books dealing with the Night Lords and they look promising. I already have a pretty long list of other books in the universe (plus a couple of already physical books on my shelf) that I am eager to read because I heard very good things about them.