“John Sargent” by Evan Charteris is available on Kindle. It is a very good read and gives some insight into his technique and methods, not to mention his interesting life. Written from the point of view of a friend of his in 1926, it gives insight into Sargent’s personality and relationships with fellow artists, including Monet. I highlighted quite a bit of it to refer back to. If you haven’t read it, I’d highly recommend it.