For what it's worth:
Yes, Le Mot et le Reste is a serious publisher. Their books can easily be found in bookstores in France.
They are the official publisher of the French translation of Peter Hook's 3 books on Joy Division, New Order, and the Haçienda.
https://lemotetlereste.com/musiques/unk ... leedition/
https://lemotetlereste.com/musiques/substance/
https://lemotetlereste.com/musiques/lha ... leedition/
I have 4 books from the Musiques series:
Pink Floyd & Syd Barrett, la croisée des destins - Alexandre Higounet
https://lemotetlereste.com/musiques/pin ... ydbarrett/
Tangerine Dream, les visiteurs du son 1967-1987 - Emmanuel Saint-Bonnet
https://lemotetlereste.com/musiques/tangerinedream/
Kraftwerk - Éric Deshayes
https://lemotetlereste.com/musiques/kra ... leedition/
Klaus Schulze, le rêve éveillé - Emmanuel Saint-Bonnet
https://lemotetlereste.com/musiques/klausschulze/
So far I have only read the books on Kraftwerk and Schulze. The writers are journalists, so the books are well written. Regarding the content, the approach is chronological, which is typical for such books. Since I know both Kraftwerk and Schulze well, I didn't learn much (except Schulze's ventures in the 80s), but I enjoyed the cultural and musical context in which the artists and their work are presented.
The books don't contain new interviews. There are quotes from past interviews.
The book about Schulze has a lot of quotes taken directly from the booklets of the SPV reissues of his back catalog on CD. So if one has those CDs and has read the interviews in the booklets, it seems a bit redundant, but can serve as refreshers...
I don't know much about Tangerine Dream so I am sure I will learn a lot of things (I have a few albums and I know about the infamous concert at the Reims cathedral in 1974, but that's about it - oh and yeah, Zeit is awesome, and I prefer Rubycon over Phaedra - yeah, I know).
Pink Floyd and Barrett, I know very well, so I am more interested in the odd anecdote and getting back to that early period, rather than in learning something new.
3 of the books feature small pictures of album covers in black and white, usually at the beginning of a chapter (the PF / Barrett book doesn't have any pictures).
There is no "nerdy" stuff, so no info on the number of oscillators in this synthesizer or on the settings on a guitar pedal for that effect on that song.
And yes, the texts are in French, obviously.
The JMJ book has about twice as many pages as the books I have, so there might be larger pictures in it. It has the same number of pages as Peter Hook's book on Joy Division. I don't have the French version but I have the US version, which is the same as the UK version only with a different cover and layout, and there are indeed several pages of photos. The French version likely has the same photos. So the JMJ book might, too.
This might give you an idea of what to expect from the book on JMJ. Since the publisher's site lists reviews about its books (see link "Voir la revue de presse" on the web page of the books, you can wait a bit after release and see what the reviews say.
Concerts: 2017: Upper Darby < 2010: Lille, Liège, Paris < 1997: Lille < 1990: La Défense