I keep reading this series because I really want to like it. The author's research is so meticulous and each period is recreated so lovingly. However, I feel as though she puts the history first and the people second. It feels as though she chooses a period in history, creates scenarios that illustrate the key aspects of that period, then makes characters move from one to the next--even if that means making them do illogical things, or things that don't gel with the values or motivations suggested earlier in the story. In this series, for instance, the heroine falls in and out of love with different men according to the needs of the plot. There's lots of internal monologue to justify that, but it never really convinces.
It also means that many plot points are implausible. Miss Lily's secret is a glaring example. She goes through decades being so utterly convincing that only one person (Hannelore) ever suspects, then Daniel realises her secret based on one gesture? Violette is another example, a character invented (I suspect) solely so she can conveniently (and unbelievably) get Sonia out of captivity at a key moment.
Finally, both book 2 and book 4 (and, I suspect, book 3) suffer from the same issue: there's a section of a few chapters where something exciting happens, but first you have to wade through the opening chapters, where the build-up is very slow, and Ms French is determined to recap the entire story-to-date in detail whether it's relevant or not (when I picked up this book, I had just read Book 2, and thought this was #3. As it turns out, it didn't matter, because books 1, 2 and 3 are so exhaustively retold). I wish the editor had been more ruthless!