Schiffer Military History has a line of short (80 page) hardcover books they call "Classic Guns of the World", and one of the recent additions to it is Llc Guillou's "The MAT-49 Submachine Gun and Preceding French Submachine Gun Designs Including the MAS-35". It really should be called just "French Submachine Guns", as it covers the entire scope of French SMG development, starting with the first post-WW1 designs through the development of the MAS-35 and MAS-38, and also the postwar development of the MAT-49 (and variations like the police models of MAT-49). It also covers French use of foreign SMGs, like the guns that came over the border in the late 30s with Spanish Civil War refugees.

Guillou's coverage is a bit shallow, given that he is limited to just 80 pages, but very wide. The information covered here is completely unavailable elsewhere in English, and is much more densely packed than the only other book on the subject, which is Jean Huon's French-language "Les Pistolets-Mitrailleurs Français". Best of all, this book is a mere $24.99 from Schiffer.