While looking around for additional sources for reytuerto's Ferré class submarines, I came across the book on the topic by Max Laubeuf himself, one of the pioneers of French submarine design and creator of the Narval.
This appears to be a complete scan of his 1915 book on "submarines and submersibles". (warning: large .pdf!)
This is obviously in the French original, but the pictures themselves are interesting all the same and understandable by everyone.
Of particular interest here are the drawings of several of the most successful early submarines, such as the US Holland (p.37), the Italian Fiat-St. Giorgio design (p.46), or the Krupp U-21 (p.98).
The rest of the text, though in French, looks interesting as well. What I could make of it includes a detailed history of the first decades of submarine design, reflections on the doctrines of submarines and submersibles for all kinds of navies, and a first analysis on the impact of German submarines on WW1 (again, published in 1915!).
So, thanks to the CNAM for curating and digitizing their collection to that extent, although they could put some work in their language skills.