I almost took this book back to the library before starting to read it. Once I started, I was oddly fascinated. But not in a way that would lead me to give a strong recommendation.
It's well written (as you'd expect from a Norton editor-in-chief). But it moves slowly, and has a 19th century feel; sections begin with a chapter of scenery-setting that's reminiscent of George Eliot. That's a bit of an anachronism in a novel that takes place in (roughly) 1916. The narrator's "this is a history, not a novel" framing gets tiresome, particularly when he brings it back at the end.
As to the story itself: despite the Romeo and Juliet remarks on the back cover, I don't see any Romeos and Juliets here. It does have a pair of young lovers, who despite their magnetic attraction, never seem to be made for each other. Yes, Toma is a better match for Harriet than her husband, but that wouldn't be hard. Potentially more interesting is the problem of invention, creation, and systems thinking. As one of the characters says repeatedly, invention isn't the issue; it's building systems. But system building ends up with Toma caught up in a patent problem that involves both GE and Nikola Tesla (who doesn't appear in the novel). And the way Lawrence handles this problem is hardly compelling; it's a minor thread that occasionally comes to the foreground. So are other potential themes, including race, class, the status of immigrants.
Race, class, and immigrants play an interesting role in the book's language. When Toma (a Serbian immigrant) is with Harriet (New England heiress of a failing iron foundry), the language is decidedly late Victorian, where a lock of hair only has to brush against skin to elicit the repressed longing and desire of a late-19th century heroine. But when he's with the mulatto girlfriend he inherited from his Black boss, the language is proto 50-shades: "she bit him on the nipple. He was immediately ready." It's an important contrast, but it never gets more than interesting. Those two worlds, different as they are, never intersect. And Toma ultimately goes with suppressed desire, rather than real passion. Too bad for him.
I'm not saying that I didn't enjoy it. This is the real Steampunk: inventors making stuff work at the start of the electrical age. But while Lightning Keeper is an OK book, it's not a memorable one.