Sin of Damnation is a great starter book into the universe. It would be entertaining to you but it's not very long. If you have already read the Eisenhorn series you could go anywhere after that.

Re: genestealers of other races. Apparently it IS possible, but improbable.

Aeldari suss it out with magic, the Tau do it with technology and the Orks just sort of Know.. but it is possible anyway.