I'm not sure if I have ever read this before, as I didn't have it in Armada. I've just read my fairly newly acquired GGBP edition, and have thoroughly enjoyed it, all the more so for it being new to me.

Lots of great characterisation along with an exciting story set in Yorkshire, for a change. The older Lone Piners are starting to grow up, with just hints of what they're going to mean to each other in the later books. The twins, too, are taking on some separate characteristics - particularly Mary - as well as doing their double act as brilliantly as ever with their enemies.

Definitely recommended, but best read as part of the lengthy series - or at least after the first few, and 'The Neglected Mountain', where the baddies in this book were first introduced.

Longer review here: https://suesbookreviews.blogspot.com/2021/11/mystery-mine-by-malcolm-saville.htm... ( )