Your Mother Was a Neanderthal #4

Thanks to The Book, Joe, Sam, and Fred are back in the Stone Age, where there aren't any math problems. But there's a tribe of hostile cavewomen, a hungry-looking saber-toothed tiger, a bone-rattling earthquake, and a woolly mammoth who wants to turn them into toe-jam. Worst of all, Joe, Sam, and Fred have discovered that writing hasn't been invented yet—so how can The Book get them back home?