This series was HIGHLY recommended by my mom (and it was highly recommended to her, by a colleague). I remember my mom devoured the entire series in just a few days, but before she started the series, she told me:
“… a friend of mine just told me about a series, she will be reading the third while she’s on vacation. The way she speaks it is a bit like our type of series, it’s called the Sign of Seven trilogy. Once I’ve read the first book, I’ll let you know what I think, and then maybe you’ll get that series also. From what she says, she can’t put them down either and she has to hold herself back to not start the third one right away instead of keeping it for her vacation.”