I have not been to Clermont or Postville but I have gone through McGregor. Went to Pike's Peak State park and then through McGregor to get to Effigy Mounds National Monument.
I see that Arcadia Publishing has a book on Clermont. Doubt if there much info you could use for genealogy but it is probably still interesting. The Arcadia books I have seen have all been pretty good.
Available here along with some other Clermont related items.
BTW - Pike's Peak was named after Zebulon Pike, same as the mountain in Colorado. Before he went to Colorado he led an expedition up the Mississippi. He saw a bluff along the river that he thought would be a good place for a fort and it was subsequently named after him. No fort was ever built there, it was built across the river instead.
Effigy Mounds is an amazing place.