Excuse-moi pour non écrire encore bien en Français...I've to continue in English.
I've come to know this story, from the series of films made for tv (I've seen it years ago on Italian tv, because it had been a coproduction), & I've loved it, so when I had the chance I've bought the original version, & wanted to check out also the books where it had been taken from, especially to know how the plot would have continued! I've read all the 5 books until here, I don't know if I'll buy also the next ones or not (probably just the next, to see an arc of the storyline concluded).
Overall, I've to say, as a balance of the love-story between the main heroes & the historical background, probabably I'd pick up the book #4 NOIR TANGO, but also on the # 3 LE DIABLE EN RIT ENCORE is really well done...
_Back to this 1 by the way, I didn't like that the writer spent too much time telling about a far away war, losing the sight of our beloved main characters: in the sense that, even if she has truly made an effort to describe (all) those foreign places, at the same time, in my opinion, she succeedes better while she manages to keep in a belieavable balance the action storyline with the rest.
As a matter of fact, the biggest problem with this specific book, is to have lost this important point, because it's not enough to use the "letters" as means (if Léa & François are divided practically the whole book as others have already said), instead of making interacting more the main roles; it's not the same to focus on minor characters we don't care about, or mostly on the political background, since this isn't an educational book afterall, is it?!
Moreover since Deforges is also good at describing steamy hot sex scenes, or passionate stories of love, families & friendship, then it's a pity when she changes completely the route.
However be aware that she's also cunning, since she had had in mind of doing a saga from the start, & obviously she always keeps out from the book a big step in the plot you're waiting for...until the next one you'll want to buy! ;-)
Generally I've also found amusing that this writer wanted to add so many famous people of the past in the path of our heores, even if more than sometimes it's also so forced to recognize it's unreal, I'd have preferred half famous names, & half not, but perhaps inspired or "close to" them, this could have been enough.
Last but not least, the language is average difficult (but not impossible) for foreigners. Have a nice read, salut