It might not seem like an original premise now, but probably when published in its original novellette form in 1957, it was. (The title then was "St. Dragon and the George.")
The story itself is interesting, quick moving, and fun. The companions are enjoyable company -- for us and for Jim. And, while the book is fun, the author manages to raise the stakes whenever necessary. At times, the book is lighthearted. At others, it's deadly serious.
It's a fun read, and, fortunately, part of a series. Don't be afraid to check it out.