Orks are my favorite faction on the rare occasions I play 40k main because they give me an excusse to shout WHAAAGH! At random intervals and then demand MORE DAKKA YA GROTTS! Fun times.
I've not read much of the tie-in fiction can you recomend a good starting point?
If you're a fan of orks, the only book still in print that I'm aware of is The Purging of Kallidus
. It tells the same story from orkish and Space Marine points of view so it's an entertaining read.
If you like intrigue, look for Inquisition books. The Eisenhorn
trilogies by Dan Abnett are great.
I really enjoyed the stories from a chaos point of view. The Word Bearers
trilogy and Night Lords
series have been excellent to see the 40K universe from a different angle.
Finally, for some light-hearted reading that's pretty funny, you can't go wrong with the Ciaphas Cain
series of books by Sandy Mitchell. His adventures against the Tau, Tyrannid, and other sundy opponents are very entertaining.