Despite not having terribly tight plotting, this was a fun read. There are lots of little side stories about minor characters in order to introduce what sort of person they were, and those stories are often hilarious. As for the actual plot, it was fun to read a book with Cardinal Richelieu as a protagonist, and the Comte de Moret was charming too. Dumas is excellent at writing vibrant, active characters.

Of course, as I'm sure you've noted, the novel was unfinished, although the translator noted that an earlier novella by Dumas provides a good ending for some of the characters of the novel. Thus, there is an abrupt shift in the book from where the unfinished novel ends and the novella begins. It jumps forward a couple of years, and those two years really matter for the characters. Allegiances have changed in that time. Major political and military action has dramatically changed the political climate. We don't get a real ending to the novel for Richelieu (although some of what ultimately transpires is explained in the novella and we see where the conspiracy ended up). In essence, the book remains obviously incomplete, but it does have a somewhat satisfying ending written by Dumas himself.

The shift to the novella is jarring, not only because of the time shift, but also because there's a serious change in style. The two protagonists in the novella are exchanging letters by dove. At first, because I was so disconcerted by the abrupt change, I didn't think I liked the novella. Upon reaching the end, however, I thought it was lovely.

I should also add that the historical character notes that the translator provided at the end of the book are wonderful and entertainingly honest.