I never seem to be able to remember when I'm reading this book if this or Ice Palace was the book I started this series with. I do know I read this before Red Demon as a major character from that one makes a return here.
Once again, Sherlock Holmes and Doctor Watson travel to Minnesota, this time to a small town out near the Red River Valley where a stone was found which was carved with runes proclaiming Vikings had been there centuries ago. As Holmes had previously aided King Oskar II of Sweden, he's retained to travel and investigate it's authenticity. Do I even have to mention that of course he's dabbled in the Runic language and it's study? Their old friend James J. Hill only makes a brief appearance but Shadwell Rafferty returns as one of the few third person investigating with them in pastiches that doesn't feel like a third wheel, narrative-wise. He works perfectly with Holmes and Watson, providing everything from information, intellect, social understanding, and even their muscle and guns.
One of the reasons I love these books is that Sherlock Holmes is taken out of his comfort zone of London and yet Millett manages to keep him in character. There is also clearly a lot of research that goes into these books and the author always provides the reader with educational and entertaining footnotes.
This is a good mystery with near death escapes and some of the best characters in the whole series. Truthfully, I've never been able to decide whether I like Ice Palace or this one more.
(Though to be completely honest, I think it's Ice Palace on both counts but on the second, it's a close call.)