Dear Lon,
Writing a biography is a very different animal from an instructional art book, so I am not sure my experience would be too helpful.
You don't say whether you are working with a publisher or editor? If so, they probably have access to the legal advice you might need. As I am not a lawyer, I can't really advise you...if you are publishing on your own, I think you really need to find someone specialized, rather than to try to figure it out independently, as your liability exposure could be somewhere you wouldn't want to be.
From an image standpoint, things I think you'd want to be sure to cover include whether the artist's surviving (spouse?) partner holds or continues to hold copyright to the artists's works (actually this is probably a question of estate law), what happens, and how you can secure copyright releases for all that you would put in your book. Moreover, you would want to investigate whether model releases are on hand and whether they would be necessary, and lastly, whether you need permission from the owners of the artist's paintings prior to printing.
I wish you the best on this endeavor, and hope you can resolve issues easily, as it is too much work and time and energy to find yourself stymied at the eleventh hour.
Thomas Buechner comes to mind as someone who has already gone through this, no doubt in spades.
Chris
PS I think this is such a great story that if you are unable to get past legal hurdles, it would make a great novel.
Administrator's Note: Thomas Buechner wrote "Norman Rockwell: Artist & Illustrator" (click title to see book).