[TOMT][BOOK] Children's book about a billion (dreams/wishes/something?) with a running joke about the population of Oklahoma

The title kind of sums up everything I remember. This was a children's book I remember loving, and it features this running joke about how the population of Oklahoma was exactly a million but it changes by a few every time someone does something. The joke is presented in intercalary chapters.

I read the book in the early 2000s, but I don't know when it was published. It's also a comedy IIRC.

Somehow, I don't really remember much about the main plot, but I know that there was some involvement of the number one billion. Maybe a million, but I think billion.