Recently I bought a book on John Nash and it came with a signed letter. Dated 16 October 1967, Nash would have been 74. It is a funny but ill tempered letter with some honesty. It seemed to be written to the artist Clifford Knight who sounds like he is trying to make contact with the artist via flattery and having it fail.
Dear Mr Knight. This book you refer to should not have been under the heading “on contemporary artists” but referred, as you rightly say, to John Nash the Regency Architect. I am sorry to say that I can not recommend any books on contemporary English watercolour painters because in the main they do not exist. There were I think 2 Penguin books, one on my brother Paul Nash and one I think on John Piper. issued about 1940 in colour. The illustrations are not confined to watercolours. In my own care I have had only had one small book done on my work in a series “British artists of today” The Fleuron (Curwen Press). Halftone plates that was in 1925 and is out of print. It is in fact rather a grievance that in nearly 50 years of work and with a large retrospective exhibition now on at the RA there is no book on my work available.
However at my age I feel it is too late to worry or care much about this! You must not take my word as gospel but I think you will have great difficulty in procuring what you need. I am sorry I cannot help further.