There are at least two books on this subject. The most recent is
called "Painterly Photography" from Pomegranete Press. It features
tons of photos manipulated this way, as well as a short,
folksy essay by the woman who did the work on how she does
it. There is also a book from the mid 1970s. I don't have the
reference at hand.
But you can also get a very useful publication on this from
Polariod by logging onto their website and sending them some