Jean Auel's Earth's Children Series, starting with Clan of the Cave Bear. An amazing series that takes place at man's early history living in a prehistoric ancient world alongside mamoths and cave bears.
I'll admit- I've only read the first 2 so far. My husband has been asking me for a while so decided to start reading the series a couple of weeks ago. I am now hooked - SO much better than the movies. I will be done with the rest soon!