The Mary Russell series is wonderful.
The Beekeeper's Apprentice (first in the series and one of my favourite books of all time) was offered as a free ebook a few months ago, on Laurie R. King's website, but I think that offer's finished. It was a badly formatted PDF, but I read it.
I think you should read the books in order, (you should definitely read Beekeeper's first), except for O Jerusalem, which should be read after Beekeeper's and within the first 4 or 5 books of the series.
http://www.laurierking.com/