I am a firm believer that airbrushing is just a tool, whereas the rules of painting apply for airbrushing as well as it does for oil painting or any other medium. Of course, there are some particularities special for airbrushing, but the basics are the same as for any other type of painting. Hence, there are a lot of youtube videos out there "how to airbrush XYZ" but most of them are not really pushing the envelopes. Instead, i found two great sources of long tutorials how to paint birds or creatures which helped me a lot recently to think about general approaches and deepen my understanding of the subject... although the artists do NOT use AB. One is from Andrew Tischler (a brilliant oil painter), the other one is from Aaron Blaise (the Disney artist). Both feature a rich fundus of long tutorials which are absolutely brilliant. I highly recommend you check them out. I don't post the links here but you will find them easily by typing in their names.