Firstly, I should say that I concur with most of the other reviewers: Georgette Heyer is one of my all-time favourite authors, and "These Old Shades" — published when she was only 24 — is her masterpiece, in the original sense of the word.
However, for the picky, I should like to add a warning about the (American) Harlequin edition. Although UK spelling is (very properly) preserved, some clown has decided to "correct" Miss Heyer's beautiful Georgian English, substituting:
p.45 — "You may lose it as you will" for "You may lose it an you will"
p.78 — "A clumsy, thick-set yoke." for "A clumsy, thick-set yokel."
pp.90 & 223 — "It is my intention." for "It is mine intention."
p.113 — "...the forward ways of the younger generation" for "...the froward ways"
p.213 — "I'm silence." for "I'm silenced."
pp.223 & 236 — "Fonteroy" for "Fontenoy", and
p. 262 — "gracefully" for "gracelessly"
But by far the biggest blunder is on p.127 where Miss Heyer wrote: "She saw the sword of the last Duke, that same that he had used in tragic '15, for King James III, and heard a small part of Justin's own adventures, ten years ago, For King Charles III."
The James referred to is of course the Old Pretender, and Charles (as the next sentence makes even clearer) Charles Edward Stuart, the Young Pretender, i.e. Bonnie Prince Charlie. But the editor (presumably after consulting a list of British monarchs) has changed these to James II and Charles II, pushing the narrative back 70 years or more!
This is of course nothing like the wholesale disembowelment that has been inflicted on American editions of Harry Potter; but if you're fussy about such things, you might want to get another (British) edition.