Mara -- I fear you will look in vain for more info on Sargent. Unless someone knows of a long lost,, and just found repository of info on his methods, about all we can do is work from the little knowledge that is already available. I have a pdf file I downloaded a year or so ago about Sargent. It is a compilation of reports, mostly from various sitters, who talk about some of the things he said and did, but that's about it. Clayton Beck (see above) has fairly well summarized what is known about Sargent's working methods.
Also, if you will check this site (
http://jssgallery.org/) you may find more information, as well a ton of reproductions of his work.
I find myself wondering about his teacher, Carlos Durand (sp) who taught Sargent the very effective method of starting with the middles, and working out from there. I have done a few googles, but find little info on Durand.