In her Bookworm on the Net blog today, Anne Weale flagged up Storycode (US readers should go here), a site which collates reader assessments of novels, and uses them to come up with recommendations for other books.
I had a go at coding a book, picking April Lady as I have just finished reading it (will Nell merit a slap? Watch this space). Having moved sliders around to give my views on how romantic it was (very), how horrific (not at all), how erotic (barely), how exotic (fairly) and which among the seven basic plots it used, I ended up being recommended Bella Pollen's Hunting Unicorns, Terry Pratchett's Mort(!), and some old thing called Pride and Prejudice.
Sylvester appeared further down the list. A bit of poking around suggested that of all Heyer's works, the only ones coded so far were Sylvester (once) and A Civil Contract (twice).
I think that this could be a really useful site, once it has a few thousand more contributors, so I strongly encourage you all to give it a go.