I've always been interested in 40k, but I have neither the time nor money to invest into the game. I see people here reference the lore all the time, and I recently found out there's a bunch of books based on the 40k universe, and they're held in high regard by the fans. What I want to ask is, what books would you all recommend to someone who is trying to break into the series for the first time?
I think having a baseline of the world is better than jumping into the Horus Heresy.
Honestly, I recommend
1d4chan. It has a ton of quick and digestible lore information. If you're familiar enough to get the humor, its much more palatable than some of the more serious lore wikis. Just to get a baseline, go over the major powers in the lore, and from there come back and ask for recommendations that sounds good to you.
If the fall of the Imperium from it's high of the Great Crusade, and the Primarchs are really interesting to you, the Horus Heresy is good, but it's like 60+ books at this point and I think really for an invested reader.
If you like the grittiness of the Imperial Guard, men no different than you with some of the worst gear facing down the horrors of the universe, Gaunt's Ghosts.
If you like some of the more silly nature of the early lore, Ciaphas Cain is wonderful.(also, not canon, but I'm in love with the retelling of an RPG campaing of "The All Guardsman Party". Despite being unofficial its a wonderfully funny mostly lore accurate piece of fan fiction"
If you want a really dark and gritty, but less war focused story, Eisenhorn tetraology is amazing, followed by the good but much less awesome Ravenor trilogy.