The Lost Future of Pepperharrow is the sequel to The Watchmaker of Filigree Street, which I re-read in preparation for this book and I think actually enjoyed more than when I first read it - at the time, I said I didn't think I'd want to re-read that book but clearly was incorrect, so make of that what you will. It's pretty safe to say that the things I found a little vexing about that book are also present in this one, hence I've given it the same rating.
Instead of London, most of The Lost Future of Pepperharrow takes place in Japan - as we start the book, Thaniel Steepleton is now pretty much fluent in Japanese and gets told he's being sent there on behalf of the British government. Meanwhile Keita Mori has been in Russia and his country is more than a little suspicious of what he's been doing, which becomes even more reasonable behaviour on their part when his role as a spymaster starts to be revealed. Mori's knowledge of possible futures is a massive asset to that role and when he and Thaniel end up in Japan, Mori is walking into a trap to try and test and/or control his powers.
The book is set at the time of massive naval expansion on the part of the Russians and the Japanese, with the latter buying a bunch of new ships for their navy from the British. On arriving in Japan, Thaniel discovers that not only has Mori been cagey about his past there, he's also married - struggling with his health, Thaniel decides the best thing to do is leave Mori behind and concentrate on his work. This is a particularly attractive option for Thaniel when it becomes clear that Mori is disturbed by his still being alive, since he can see a number of possible futures where that's not the case.
All of this relationship drama is happening alongside all sorts of electrical experiments which are creating 'ghosts' of past and future events, mimicking what Mori can do with his mind and throwing the local population into turmoil. There's plenty going on here and Mori is at the heart of it, unsurprisingly, having set into motion a chain of events that we later discover is mostly aimed at preventing an all-out war Japan is likely to lose at that point and also helping find a cure for Thaniel. At one point, Mori is believed dead and a former friend of his tries to pin his murder on Thaniel, before fortunately everything is resolved (though not without loss of life).
As with the previous book, the one area in which the author falls down a little is still the characterisation of the women in her stories. While Grace Carrow, who played a major role in The Watchmaker of Filigree Street, also appears here, it's Takiko Pepperharrow who is the main female character - she's a little better fleshed-out than Grace was, so it's a shame how things play out for her when there seemed to be other directions it could have gone. Anyway, if you can get past the role of woman as barrier-to-relationship where our protagonists are concerned, then you'll probably like this book as well.