I like these books; the first one had some plotting problems towards the end, but this one offered a much tighter and surprising story. The author does an excellent job with atmosphere and setting too, although I can't comment on historical anachronisms. Penrose does include an author note at the end discussing the backdrop of the story and offering some non-fiction titles for further reading.
There's a stronger element of romance to these books than there were in the previous historical mysteries by Penrose, but it's not at all overbearing, and the characters are much more sympathetic. I was worried the author was going to drag Charlotte's big dreaded secret out even longer into a 3rd book, but she pulled it out right at the end (and spoiler - it's not even a little shocking). I continue to like the two waifs Charlotte has taken under her wing too; I generally don't like kids in my mysteries much, but they work here and they're never purposefully cute or cloying.
I'd put this series in just about the same class as the Lady Darby series by Anna Lee Huber - so if you like those, you might enjoy these.