EDITED TO ADD:
MaryK got it right! It's Thunder on the Right. Here's a quote from an article that Mary Stewart wrote in 1970:
With Thunder on the Right, I tried a technical change of approach, from first person to third. I had dropped naturally, without calculation, into the first person ... but now thought it right to experiment. Of course writing in the first person has certain drawbacks, especially in "danger" and "suspense" situations--certain elements of surprise are cut out, the viewpoint is limited, and direct action is also limited to scenes where the protagonist is present--but for me the advantages far outweigh the losses. The gain in vividness, personal involvement and identification is immense.
In Thunder on the Right, with the third-person approach, I found I had more freedom of action and viewpoint, but in my next novel, Nine Coaches Waiting, went back with a kind of relief to the first person, and have used it ever since.
---"Teller of Tales" in The Writer, Vol 83, p.11