Oh Hyacinth. Hyacinth, Hyacinth, Hyacinth. So if there exists anyone in the world who wasn't chomping at the bit for Hyacinth's book since waaaay back in The Duke and I when she calmly announced to Simon Bassett that if he didn't marry Daphne, he should wait for her, then, well, I don't want to know about it.
Hyacinth is every bit as brash and brazen and fucking fantastic as she has been throughout the entire series. If that wasn't enough, not only is Lady Danbury once again a major character with tons of page time allotted her to be perfect, but the male lead is Lady Danbury's grandson and!! is actually worthy of being so. Gareth takes his place on the side of actively likable male leads in the series, mostly by being just as charmed by and loving just as fiercely as we do all the correct ladies.
The structure of It's in His Kiss is a of bit a deviation from previous books as there's actually a nominal plot outside of the Hyacinth/Gareth romance, which manages to lead to utter delights like their multiple counts of sneaking around at night and breaking and entering with bonus crossdressing. Basically, a romp completely perfect for Hyacinth, whose daring and desire for adventure is nicely set off by her unwillingness to ever be off-balance or have people think she's not good at something.
She and Gareth are wonderful as they try to one-up each other and push and pull as they navigate their growing feelings. More than any other book in the series, even with Gareth's dad angst, It's in His Kiss is light, irreverent, and perpetually fun.
That lightness, I think, veers towards preventing it from having some of the emotional gut-punch quality present in some of the other books, but it still easily ranks right up there with Romancing Mister Bridgerton and To Sir Phillip, with Love as the best of the series.