Disclosure up front: author T.E. MacArthur and I co-chair the author programming at a local convention.
There. Now, that's out of the way.
In this the fourth volume of MacArthur's Volcano Lady series, the star-crossed Tom Turner and titular Professor Lettie Gantry are both after the same man: the one the English newspapers call the Earthshaker. He claims to have created a weapon that creates earthquakes on demand, and plans to sell it to the Prussians. Tom and Lettie are working separately, unknown to the other, to stop the madman.
Throw in an homage to Jules Verne's "Journey to the Center of the Earth" in the person of Professor Otto von Lidenbrock, and you've got the basis for quite an adventure.
Taking place in England and Iceland (where MacArthur has traveled extensively for research purposes), the adventure tale examines political science, geology, geography, and yes -- even the nature of love.
This book is simply delightful.