I read The Saga of Pliocene Exile by Julian May, all four books in a row.
It's a really crazy read, just the way I like it.
A group of people from the future are sent through a time portal straight into the Pliocene Epoch (the same era where the dinosaurs are gone and human ancestors still look like apes). And this is not a scientific expedition, but a group of misfits who have no place in their own world and time. But in the Pliocene they meet two alien races who came to Earth at that very time and settled there.
These are very unusual books, and I really wish I had found out about them earlier, and that the author had already died.