How does evolution produce complex organs
The best introduction to evolution of complex phenotypes via master control genes is Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom by Sean B. Carroll. You might also want to have a look at a more recent Evolutional developmental biology (Evo Devo) book like those you get when searching for "Evo Devo" in Google Books.

In summary, it is possible for a mutation to change a shape by changing the spatial distribution of a transcription factor during embryo development. What's more, by changing the distribution of two mutually dependent factors, formation of any shape is possible if the distributions overlap. Such transcription factors are highly conserved in evolution and the most important cause of the endless forms we see.