Here's another of Joan Aiken's "ripping yarns"---no one could write this sort of adventure story like she did! The plot is summed up in the editorial review above, so no need for me to go into detail. I'll just say that it's a very enjoyable read, certainly. I enjoyed the construction of the story: the book has a bit of the feel of the early Victorian magazine serials. In fact, the style of the first-person narrative put me very much in mind of Dicken's "David Copperfield." Each chapter is almost a self-contained adventure of its own, another step in Felix's journey from his grandfather's home in Spain, to his paternal father's family near Bath. (It's unfortunate that I know almost nothing whatsoever about the geography of northern Spain. I think if I had been better able to imagine what sort of landscape Felix travels through, it would appear more vividly in my imagination. Aiken perhaps is not a writer given to lengthy descriptions of the scenery. But that's not really a complaint about the story at all, just an observation.) The novel certainly is a exciting yarn---such adventures Felix has! While this book is a complete tale in itself, once you have finished it you will surely wish to read more about Felix; his adventures are continued in "Bridle the Wind."
I wonder if this story is, in fact, set in a sort of "alternate history" universe just like the Aiken's "Wolves of Willoughby Chase" series was. Because I found it interesting that Felix, throughout the journey, reads passages from a book of his father's titled "Susan." It becomes obvious that this book is, in fact, none other than a Jane Austen title---though in "real life" Austen did not have a book published by that name. It isn't spelled out at all---I daresay a lot of people reading this book wouldn't give a second thought to what Felix was reading---but I got a kick out of noticing that...
The dust jacket illustration on my old copy is very mediocre, though---a rather dull painting of a boy riding a mule by the seaside on a sunny afternoon. Honestly, it makes the novel look like some sort of animal story, not a hair-raising adventure at all. I see that these books are going to be reprinted soon, and the new paperback designs are considerably more exciting and dynamic than the old hardcovers were.