I agree with Anoctris. Any of the books written by Dan Abnett are an excellent intro into the Warhammer universe. I would personally recommend the Eisenhorn trilogy as that's what got me into the universe to begin with.
The Ragnar Blackmane books are excellent as well, though written by William King and focused around an entirely different part of the universe (Space Marines instead of unaltered humanity).
If you're looking for back story on why the humanity is the way it is in the universe, the Horus Heresy series is an excellent source.
Really, there are several good entry points, but I do think that either the Eisenhorn trilogy or the Gaunt series is the best.