Best book to begin the Warhammer 40K universe?

tons of great places to start, some suggestions:

Eisenhorn series - spectacular, focuses on inquisitor and his retinue, get a good sampling of his interactions with various races

Gaunt's Ghosts - more standard military sci fi, focusing on a commissar and his unit

Ciaphas Cain - a more action adventure along with some comedy (which is slightly rare in 40k) again focusing on a commissar

Any of the space marine battles / anthologies can be good depending on which faction(s) you like.

Are you more interested in 'good' factions, evil factions? regular humans and their struggles or space marines / aliens etc?

Its a fantastic universe with tons of variety

If you want to go hardcore and get a good idea of the basis of the current events, you can start the horus heresy novels, which are currently running about 60 deep I think, they focus around the huge imperial civil war that took place 10,000 years before the 'current' timeline so ~30k

historical = horus heresy novels, they start off with the story of the primarchs, the original space marine legions and the huge civil war that led to the events that are happening in the main 40k novels.

The Eisenhorn and Ravenor series also have a ton of detail on imperial politics as well as just being fantastic books in general.

If you don't want to get bogged down with 55+ books in the Horus Heresy, I really don't think you can go wrong with Eisenhorn.

Also if you see any of these names as authors, you have a pretty damn good shot that the book is a good one:

Dan Abnett, Graham McNeill, and Aaron Dembski-Bowden

Thanks for the advice. I think I'll start with the Horus series since it seems to explain the history of everything.

I 2nd Gaunt's ghosts. I want to say it's like 12 books but It's a nice different perspective from the Space Marines.