“Actually, no,” Holmes said, completely ignoring the man’s fury and sounding merely bored—an old and effective technique of his. “She will not wear those clothes, or anything like them. No burkah, no bangles, no veil. She will not walk behind us, she will not cook our food, she will not carry water on her head. This is not, you understand, my choice; I should be perfectly happy to have her clothed head to foot and in a subservient position—the novelty would be most entertaining. However, she will simply not do that, so we must either live with it or separate. The choice, gentlemen, is yours.”
In O Jerusalem, we go backwards, to the period between The Beekeeper's Apprentice and A Monstrous Regiment of Woman, and get the story of Russell & Holmes and their flight to Israel after the attack in London. Importantly, we also meet the Hazr brothers, Mahmoud and Ali, who figure prominently in Book 6, Justice Hall.
This book does not end up among my favorites. It is really more thriller than mystery, and while I ended up quite attached to the Hazr brothers, the setting is not as engaging (to me) as the books set in England. It is worth reading, if only to fill in some gaps and to meet the brothers.
I did enjoy the conflict between Mary and Ali, and the slow, grudging respect that the brothers developed for her. Mary Russell is 100% badass, and this book showcases that badassery better than, possibly, any of the other books. Her knife throwing skills, her ability to survive, and just her endurance, her courage, and her tenacity, are demonstrated by the events in this book. She isn't cartoonishingly drawn, she isn't a superhero, but she is heroic, and she is tough.
So, I recommend it for completists, but wouldn't suggest it as the book to read to be introduced to Mary Russell. And, if one is to read Justice Hall, it is essential that this book be read first. Justice Hall might still be enjoyable, but the depth of the characterizations will not be comprehensible without first reading O Jerusalem. And Justice Hall, in my opinion, is the best of the first 6 books.