I recommend to visit the following websites too:
Pat Harrison ( Great Paintings in Oil and Acrylics)
Andre Jute ( Best Seller Author and Painter)
Russ Stutler ( the semi god of urban sketching)
Matthew Mattingly ( great art by an all weather artist)
Brian van der Spuy ( artist and author)
Michael Daley‘s very informative website about restoration and conservation malpractices
Discuss about sketching at sketching forum
Graham Beech ( for excellent website hosting and technical support)