I learnt my weathering from a book called "The Art of Weathering" by Martin Welch, and my airbrushing from "Painting and Lining in Smaller Scales" by Steve Barnfield, followed by much practising and adapting to suit my needs. I've just purchased "Airbrushing for Railway Modellers" by George Dent to bring me up to date with the more recent developments in paint and airbrushing.
I continue to learn, pick up tips and be inspired from the sklilled modellers on this and other forums.
Ian