While waiting for a full review of the book, I though I'd post a quick word about it (esp. because it's discounted at the moment on Amazon).
I have the French edition of Welles at Work (which I assume is exactly the same as the English one), and it's quite a great book.
While there is a lot of information that can be found elsewhere, scattered in various sources (interviews with Edmond Richard, for example) , this book does a great job collecting it all in one place, as well as presenting a great deal of other info I've never been able to find elsewhere.
Each chapter covers a specific film (including Fountain of Youth),and focuses on the technical aspect and production: the phototograhy (there is some mention of filtration and lenses), a bit on editing techniques, a lot on set design, and a lot of info on how the productions were organized and shot.
For example, the authors have assembled tables listing the exact contribution of the various DPs on Ambersons and some information on the various DPs on Lady from Shanghai.
For Touch of Evil, there is a nice description on how some scenes originally meant to be shot in the studio were instead done on location (which of course is what gave rise to the complaint about Welles "changing his ideas in the middle of shooting").
For The Immortal Story, there are small thumbnail photos comparing different shots made with the various-sized lenses used for the film.
There is an embarrassment of riches in terms of photos and documents: a make-up test of Welles for Heart of Darkness, memos between Welles and Richard Wilson during the post-production of Lady from Shanghai, as well as some pre-production costume and set drawings, and location scouting photos.
Overall, a well-researched and rich source of information, and probably a must-have for anyone who posts here.