Okay. If you're reading/have read Louise de la Vallière, you are DEEP down the Musketeers rabbit hole. Nobody picks this one up at random and thinks, "hey, I'll choose this 700+ page book in the middle of a series for some light reading."
At this point, fellow Muskereader, you are committed.
I'd read a few spoiler-free reviews before this, and did some light investigating into the plot beforehand, so I knew that out of all five books, this is the one where our beloved Inseperables are seen the least. They show up here and there, making cameos that have you squealing in delight: "the fellow who just punched those bad guys and took his hat off is d'ARTAGNAN!" or "oh, the sneaky fellow who swept off his cape is ARAMIS!"
The rest of the focus is largely on King Louis XIV, France's legendary Sun King, and his court, at the height of his blazing youth, and at the precipice of his power. The royal romances and secrets become the main ingredients of this story, and the original Musketeers take a back seat to palace intrigues.
So, I was going in with the view that this one would have to be endured with gritted teeth and determination.
I had SO. MUCH. FUN.
Palace intrigues turn out to be far more interesting than palace politics, and I'll take passionate declarations and stormy jealousies over financial discussions any damn day of the week. A friend (hi, Papercuts1!) who is buddy-reading along with me likened the 200+ pages of trysts in the dark to "Shakespeare on weed" and I laughed out loud because it's so true.
The title character, Louise de la Valliere, is actually the least well-developed character. She's very much a girl from her time, and can't be blamed for powerlessness, but she consists mostly of moralistic, heavy-handed speeches to her friends, crying and wringing her hands, and fainting fits. I'm almost tempted to go back and count a) the times she fainted, and b) the times her only dialogue consisted of, "Oh, sire!"
However, she's the title character for a reason: she's the still point at the revolving centre of the court. King Louis falls in love with her, and that has far-reaching repercussions. Raoul (Athos' son) and his best buddy de Guiche (himself in love with Louis' sister-in-law) are the next generation, and unwittingly become drawn into the web of shifting alliances because of her actions. The Comte de Guiche is particularly Musketeerish; he's involved in a pistol duel that had me on the edge of my seat, and he provides the swashbuckling we so badly miss from the other books.
Elsewhere, Aramis, master of secrets and swirling cloaks, is slowly setting the stage to supplant the king with the Man in the Iron Mask, a mysterious prisoner of the Bastille. He's super sneaky, collecting power in all kinds of unsavoury ways, betraying quite a lot of people and lying smoothly through his teeth in order to serve his own needs.
Overall, this book sets the stage for the final chapter, and as much as I thought it would drag, I found life at court, with its rich atmosphere and intrigues, was fascinating. If you want romance, you'll certainly get it: King Louis is deeply in love and he makes you believe it with some fantastic, swoon-worthy dialogue. When the original Musketeers do show up, they provide the novel's most interesting moments: Aramis' scheming is scene-stealing and intense, and d'Artagnan is the hero that everyone is going to need by the time all this is over. Athos is missing for most of the book, but his bromantic moments with d'Art at the beginning are heartwarming as always, and Porthos, the lovable giant, injects some much-needed humour.
I actually think this one captures the Musketeer spirit more than the previous installment: forbidden duels, secrets aplenty, trap doors and moonlit intrigues, passionate declarations of all kinds and d'Artagnan heroically making his way through it all.