This is an interesting idea - presumably the lack of effect on the timeline is the universe working very hard to prevent a paradox. Or maybe there's now another timeline where Willow had to buy another copy of a lost book, spent her allowance on that instead of something else, and as a result the universe is comprehensively different - maybe she never met Tara or something. Or is this the universe where she never met Tara, and not having the the book led to them meeting somehow?