Many books are published in country-specific versions, though I think it's more common now than it used to be.
When I was younger, reading Douglas Adams books from the library, they were all British. Copies I purchased new, several years later, were only available in American versions.
My copy of Dirk Gently's Detective Agency, which I bought used, has British spelling.
My first-edition Good Omens is British (Adam reads under the covers with a torch), but my friend's later copy is American (he reads under the covers with a flashlight). At one time, I had both versions of American Gods, though I passed my British (softcover) version to my mom and kept my American (hardcover) version.