The prior review by "Harriet Klausner" is plainly by someone who has not read the book. At best she had read the jacket notes and merely rewritten them. Her description applies to about the first three chapters of the book. There's lots more after that.
Anyway, the book is written for a young adult reader. As such I'd call it a decent read. Probably a 5-star read if I were still a teenager, but the characters and plot were perhaps a little slight and the idea not enirely original. But there's some interesting historical background which an American kid would never get in school but which would be stock stuff for an English kid.
The basic idea is, at first, introduction of advanced technology (rifles, airships, etc.) into the English Civil War. Turtledove tends to get credit for this idea (see his Guns of the South), but the idea's been around since at least the late 1960s and was perhaps best done by de Camp in his Lest Darkness Fall. However, in Jeapes' book the advanced tech doesn't come from the future, it comes from... well, you'll have to read a bit to get the first glimpse of that. Read on and you'll figure out exactly where it is that "John Donder" et al come from.
Donder isn't the only main character of the book. The lost son, whom he finds right away, is just as important. And as major supporting characters we have the Stuart royal family, Cromwell, Monk, and others. Also Donder's compatriots from wherever it is that they come from. Much of this book turns on John Donder's conflicted loyalties and on the efforts of the Englishmen to overcome their divisions and fight off the mutual enemy.