I've read the second and third books in Tasha Alexander's Lady Emily Ashton series out of order, and I wish I hadn't - not because A Fatal Waltz built directly upon A Poisoned Season but because the latter was a much more tightly plotted novel. To be honest, I was a bit disappointed in A Fatal Waltz, and if it weren't for my slight obsession with reading in order (and the fact that I'd already bought the rest of the series), I might have just abandoned Lady Ashton.
The titular season is the London social season - an exhausting and expensive whirlwind during which the rich and the well-born try to save the estate or buy some class through an appropriate marriage. Emily has recently come out of mourning for her late husband and is attempting to navigate the season on her own terms, accepting only those invitations that interest her and not being maneuvered by her mother into another marriage with a near-stranger.
Alexander introduces four threads which unravel Emily's plans. A man claiming to be the son of Louis XVI arrives in London, charming the ton and becoming the main prize of the season's marriage market. Soon after, someone poisons an acquaintance of Emily's. As she investigates his death, Emily finds that she is being stalked and that someone has begun to spread scandalous rumors about her. Alexander ties the four threads together quite neatly, and in an unexpected way - she didn't quite "get" me, but I didn't guess the identity of the murderer until a few pages before Alexander revealed the killer's identity. Alexander also gives her supporting characters - Emily's suitor Colin Hargraves, her childhood friend Jeremy, Duke of Bainbridge, her bluestocking American friend Margaret, and Frenchwoman of a certain age Cecile - enough depth that their romantic entanglements and exposition scenes add to rather than detract from the mystery.