So I've been thinking about starting to read WH40K books. Where's a good place to start from? I'm sure there's quite a few books out there now and I don't know if they go in order or not really.
I recommend the Horus Heresy series. You get too see how Primarchs, space marines and regular imperial citizens all act when they encounter each other. Some of the most interesting things a author can write about is what these guys do when there not on campaign. Plus bolter porn!
Depends on what you are interested in. I recommend starting with the Eisenhorn trilogy or Gaunt´s Ghosts novels to get a feel for the Imperium and the wars being fought from a human perspective before moving on to Space Marines.
Horus Heresy Ciaphas Cain Ahriman Gaunt's Ghosts Space Marine Battles The Eldar Path Night lords/Ironwarriors/ultramarines omnibus These are the best that I have read so far.
I second Warsmith Dantioch's suggestions. That said, I would urge you to hold off on the Horus Heresy and Ciaphas Cain books until a much later date. They're good to be sure, but you'll enjoy them a lot more once you gain some real context to the universe as a whole. I would also add the Word Bearers omnibus as a suggestion, but if you're looking for individual, isolated books rather than series, then I recommend Storm of Iron or Rynn's World. They're both fairly open to new readers and give a good, relatively balanced insight into the setting as a whole.
Once you read a few SM battle books I recomend picking up the the deathwatch xones hunter book. It will ruin all other SM books for you imo.
If you go into the Horus Heresy I think that the official reading order is a bit off, start with the original 3 then read First Heretic because it's chronologically the first in the timeline and provides context for Chaos. Besides that I thought Lord of the Night makes for a good starting point, it has a lot of detail for a Hive World's Imperial Society and how things have changed from the time of the Horus Heresy from someone who was alive during the Heresy but wasn't around for the actual changes.