I asked my friend and the President of The Dakota Institute of the Lewis & Clark Fort Mandan Foundation in North Dakota, David Borlaug, to personally recommend two new books for you:
Both are available at www.FortMandan.com, along with many other books and DVD's. It is well worth your time to take a quick perusal and order today.
Last years picks can be found HERE and it's quite a nice collection. Please check that out too.