I don't think that we use "this past weekend" very much (in British English at least). I would interpret both these sentences in the same way, but I would only say 2.1.This past weekend I read a good book.
2. Last weekend I read a good book.
I agree. It sounds normal to me in American English, but not always absolutely interchangeable with last weekend.I hear "this past Monday" or "this past weekend" quite frequently in the US.