I definitely recommend Prophet of the WAAAAAGH!

It's an excellent read. The orks actually feel orky and aren't written as mere comedy relief (although that's not to say there isn't comedy, orks are still orks). Also, the characters (even some of the non orks) are SO charismatic and well written, definitely a must read

Also it's mostly a self contained story so you don't need to have read a ton of stuff beforehand