Hi from Paris,
Apart from our typical genealogy researches, you can find a lot on "your" Simon Kra and his family.
Unknown from me five minutes ago, I entered "Simon Kra " to search French Google : never forget Google at a search's start !
I found he had been an important book editor in Paris before 1939.
As his children are named in these articles, we can be sure it's "your" Simon.
A full article had even been written on him in French Wikipedia :
and you can access some others.
He wrote 2 or 3 books.
You can use direct translation to English if your French isn't ... nobody is perfect.
On another point, I checked French white pages : more than 150 persons are named "Kra"...
I search some others clues.
Archives & history of Medem Center - Arbeter Ring (Bund / Workmen Circle of France)