There are lots of ways to approach this. Having a set of rules adds to the fun by keeping options manageable when a lot of choices are available, and adding to the challenge when they are less so. Here are my rules. No judgement on any authors or works should be inferred from the rules; they’re just one way of setting boundaries. I keep them in mind, bend them when I want to or have to, then go with whatever appeals to me. For other ideas, check out other people’s sites.

Rule 1: I started by saying that the book had to be fictional. I recently heard an interview with Michael Holroyd in which he said he preferred “creative” and “re-creative” instead of “fiction” and “non-fiction”, and I like that. So, it has to be a creative work: a novel, short story collection, play, epic poem, or poetry collection, in that order of preference. When necessary, even a single short story will do. (In one case so far I have resorted to a different “creative form”, the essay).

Rule 2: The definition of a country generally follows the United Nations list of "sovereign states", occasionally supplemented by internal political entities often considered “countries” and some disputed or partially recognized states. When grey areas or controversies arise, it’s best to read one more book. 

Rule 3: The book must be set at least in part in the country in question (even if it is in a person’s memory, a fantasy version of that country, or some such variant)

Rule 4: The author must have spent a significant portion of their childhood and youth in the country, even if they were not born there or if they later left. The idea is that the author’s view of the world was formed at least in part in that country.

Rule 5: The temptation to read the best, the greatest, the most important book from the country’s literature must be avoided! Such books should be included; so should detective novels, science fiction, humorous works, etc., (as long as they are well-written).

Rule 6: Each book I read must be from a country adjacent to a country I’ve already read. I actually started with two countries (the Czech Republic and Ireland) and I’ve been spreading out from these. This provides some structure, adds to the fun and challenge, and also gives some sense of cultural continuums and transitions as one moves through literary and geographic space.

Rule 7: Keep it fun!