Found in 2 comments on Hacker News
hbbio · 2019-06-06 · Original thread
Note that the works seem to be in the Public Domain, since the author died in 1934 and the copyright law is 70 years after death in France. However, there is a co-author but no information is available on her/him which might extend the copyright duration.

Another argument for public domain is someone translated it to English and plans to sell it (at least on Amazon):

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