Reddit mentions of Ravenor: The Omnibus

You talked to the right guy! If you want an awesome point of view and introduction (or just great stories if you are already a fan) to the Warhammer 40k universe I suggest you read the following in the order I put them:

Eisenhorn Omnibus

Ravenor Omnibus

The Emperor's Gift

All these are intertwined, there is another trilogy to follow up Eisenhorn and Ravenor but only the first book has been release for that so far. The Eisenhorn Trilogy are three of my favorite books and I read from many, many sources outside of WH40k

Any of these three series I recommend as modern-day Lord of the Rings classics

Eisenhorn, books 1-3 of a 9 book series are simply the best books I've ever read - the series is currently on book 7. I recommend Eisenhorn to anyone that just likes to read. Books 4-6 are just as good, called Ravenor, with the 7th book in the series now out, called Pariah.

Another amazing series of books I love are the Old Man's War books, effectively a series of 6 books with some short stories in between and the last book being broken up into thirteen short stories (such was the demand for the books while they were being written). Fantastic, fantastic series - I recommend them to anyone.

I also love and recommend The Dark Tower series, 7 books. They're the only thing written by King that I've liked.

Pick up a few of the books. I would read, in order:

Caiphas Cain, HERO OF THE IMPERIUM

Gaunt's Ghosts

Eisenhorn

Ravenor

Those 4 will give you a fairly firm grounding of the lore and a feel for the universe. The first two are war books, the latter two focus on the inquisition.

Well, you raise an interesting question when you say you're after "book 'one' of the lore". I don't actually think there is such a thing.

See, 40k is more an environment, a genre, than a story-arc. So much goes on that is totally independent of everything else. And, in fact, with the Horus Heresy stuff a lot of it even happens in a totally different timeline. An analogy would be like asking for book-one in the Star Trek universe; you could pin down the very first thing that ever happened in terms of date, but it would bear zero resemblance to a lot of the more popular, better writing.

Now, I've been kicking around 40k fluff for, god, getting on for thirty years. And for my money the best place to start would be a set of books based around the Inquisition - since they're the ultimate enforcers of the Emperor's will in both Civil and Military matters they see rather more of human society than merely the battlefield. If you only buy one book make it the Ravenor omnibus by chap called Dan Abnett (he also wrote the Gaunt's Ghosts novels, as well as being one of the main writers of the Horus Heresy series, so he knows what he's on about).

The Ravenor books contained in this omnibus give you three things:

1. A deeply interesting take on Humanity in the far future
   
2. A seriously violent, seriously action-packed read
   
3. A very, very good grounding in all of the various factions of the Imperium and, too, the evils that face them
   
   However, saying that, there's a few bits of free stuff kicking around the internet that are worth reading on their own merits. They're showing their age these days but hell, so am I:
   
   The first is Deathwing, a short-story written as part of the release of Space Hulk; this was a board-game based around Space Marines in Terminator armour assaulting huge, ancient spaceships that appear every now and then, emerging from the Warp with no warning. In an ideal world they're empty and stuffed with delicious technology. However, a lot of the time they're filled with nasty surprises; hence the Terminators and their storm-bolters. Games Workshop are pretty good at deleting anything from the internet that violates their copyright, but a .pdf of it is here.
   
   The second, though, is more important, and is basically as holy a piece of writing as you'll ever get in 40k. It's the story of the final, monumental, universe-deciding duel between The Emperor and his twisted, warped son Horus. Again, GW do an inquisition-grade job of purging it from the archives, but a copy of it is here, if you can excuse the advert at the start. Thinking how the Horus Heresy series will end, and how they'll revisit this, sends a shiver down my spine.
   
   Anyway. Wall of text over. I hope you enjoy whatever it is that you choose in the end :-)

Eisenhorn is probably the best possible place to start in 40k. You get every possible angle of 40k explored in this omnibus as well as a some fantastic characters. Add Ravenor for flavor.

After that, dive into the military side of things with Gaunt's Ghosts. This covers primarily the Imperium vs Chaos side of the conflict, but also the internal conflicts the Imperium suffers from.

Now that you're thoroughly acquainted with the human side, view the universe through the eyes of the super-human Space Marines in The Ultramarines Omnibus. This series shows the strengths (and weaknesses) of the Space Marines as well as introducing the terrifying Chaos Space Marines more thoroughly.

Cry for death to the False Emperor with Soul Hunter. Follow up with the sequels in Void Stalker and Blood Reaver to get a very interesting perspectives from the Renegades.

Finally, finish your introductory odyssey with The Horus Heresy. You'll be reading how the modern universe came to be.

This is a "fuck-ton" of reading and will keep you busy for 6 months to a year or more before you finish. These series are the best of the best in the novel side of things. There are huge amounts more in the Army Codexes ^^that ^^you ^^should ^^be ^^able ^^to ^^find ^^for ^^$0 ^^on ^^"sharing" ^^sites and other excellent novels. PM me anytime and I'll happily answer questions/suggestions.

The Eisenhorn and Ravenor Series by Dan Abnett from the Black Library.

Books! Especially the two Dan Abnett trilogies: Eisenhorn and Ravenor.

http://www.amazon.com/Eisenhorn-Warhammer-40-000-Omnibus/dp/1844161560

http://www.amazon.com/Ravenor-The-Omnibus-Dan-Abnett/dp/1844167372

Each is an "omnibus", three relatively short novels that were combined into a single book (750 or so pages paperback each?).

Start with Eisenhorn, it's a great primer to the general 40K world. Ravenor is much darker, and also fantastic.

Oh my God, it's such good stuff. Pirate the epub versions, or buy a used copy for pennies somewhere. I think they're out of print now.