Behold, Here's Poison is the sequel to Death in the Stocks. It just so happens that the two books are more or less identical in many ways. They also both happen to be the best of Heyer's mysteries that I've read so far.
In Death in the Stocks, a well-to-do and generally disliked businessman is found stabbed to death and his body displayed in the old town stocks. In Behold, Here's Poison, a well-to-do and generally disliked businessman is poisoned to death. In both books, family secrets are unearthed during the investigation. There are lots of other parallels, but I don't want to spoil anything.
They're formulaic, but they're excellent entertainment. Both feature Inspector Hannasyde, although these aren't really sleuth-oriented books in the modern sense. The mystery in Death in the Stocks is particularly good; I wasn't able to guess the murderer at all. I didn't get the chance to guess the murderer in Behold, Here's Poison since I'd accidentally read a spoiler a few months ago that tipped me off, but the plot was well-done and full of surprises.
Heyer really shines when she's writing dialogue. In particular, the banter between the brother and sister in Death in the Stocks is often hilarious. I'm also always surprised at Heyer's ability to make me hate a character early on and then bring me around to liking that same character without actually changing the things about the character that annoys me. For instance, in Death in the Stocks, at first I found Antonia irritating and kind of bitchy, but after a few chapters I liked her and by the end I was cheering for her. In Behold, Here's Poison I loathed the smooth, caustic character of Randall for half the book, then gradually started to like him.
Heyer wrote a lot more mysteries, but I suspect these are her best. I'll be reading more of her mysteries (I can't stop myself), but not right away. I think I'm reaching saturation point for the moment.
B&N link: Death in the Stocks
B&N link: Behold, Here's Poison