There is a new Fabergé book in case you haven't noticed it yet. Ulla Tillander-Godenhielm published "Fabergé and his Finnish masters" in 2008. It was available only in Finnish.
Now this book has been translated to English and published by Unicorn. ISBN 1-911604-20-1
I recommend it even if the chapter regarding marks is rather "thin", instead the biographies are interesting reading.