This was an interesting exercise. I had heard that Van Gogh was a regular correspondent with his brother, but the sheer volume of correspondence took me by surprise, well over 900 so far as I could see.
I found the following site the best repository of the letters
http://vangoghletters.org/vg/letters.html
Reading through the letters gives a real sense of period and Van Gogh’s powers of observation are remarkable.
Attached is a sheet of a few extracts from letters and my comments and learning points from them.
Whilst researching this subject I also came across this fascinating account of how Van Gogh’s use of colour was influenced by the time he spent in Paris and his exposure to other artists there.
http://www.bbc.co.uk/news/entertainment-arts-24537244
This was a good exercise, without it I would not have the same insight that I now have to Van Gogh’s approach to painting.