The first mixes a little fiction with a lot of historical while the second mixes a good bit of historical with really great fiction. This is the only way I can come up with to explain it. Eckert tells a factual and historical story using a bit of fiction in these books. Sorry I am having trouble expressing myself here. Everything in the book that happens is fact.
Every incident is something that occurred and everyone in the book existed.
These people, places and events are documented in some way or another. Mostly found in letters and jounals. The only thing fictitious about it is dialogue. If Eckert found a simple statement in a letter from Joe that while on his trek through the woods on April 2, he came across Pete, Eckert puts this in the book and using artistic license creates what the conversation they might have had. You get the picture I hope.