Apple's store systems are country-based. Plus, books are a very quirky market. Major titles will be available everywhere, but for anything else, it is very difficult to cross borders.
There is an easy solution. All you need is a credit card (and possibly home address/phone number) from the US (or UK, Canadian, other English country). Create a new Apple ID using that credit card and a new e-mail address. Then you can use that Apple ID to get books for that country. But remember, it's still country based, not language-based. Don't expect to read any Canadian authors anywhere but in Canada.
It has been a few years since I had multi-country Apple IDs. It can be done. But it may complicate your life somewhat. If you just restrict it to Apple Books, then it shouldn't cause too much grief.
Another option is also to get ePub books elsewhere and open them in Apple Books.