I really enjoyed this. I was worried that I wouldn't find it as interesting or engaging as the ones about the Mysterious Benedict Society, but I did. Everything was clear and believable (though the awful-orphanage is a bit overdone, it is, of course, the most common form given selfishness and the general poverty of such places), though I did find the mystery a bit obvious. My first thought on reading about the treasure was that it would be (view spoiler)[the library (hide spoiler)], though of course I didn't know what had really happened with the inheritance. (view spoiler)[John's adoption by the Hopefield's didn't surprise me in the slightest, nor did Violet's deafness. I was expecting the machine to come out of the mine in time for the mining company to have to pay, but dismantling and rebuilding it did not occur to me. (hide spoiler)] 
 I'll definitely be looking out for other books by this author, which will hopefully include some more of the Mysterious Benedict Society, despite the fact that (view spoiler)[Mr Curtain is out of commission as a villain (hide spoiler)].