I bought "Along the Trail with Lewis and Clark" by Fifer and Soderberg before we started following the trail on a 2013 trip. Good reference and timeless. There's a handy National Park Service map called "Lewis and Clark Trail", too, part of the National Historic Trail series.
We started at the Arch in St. Louis and enjoyed the excellent museum and films underneath. There's a statue down by the river near the Arch of L&C. We stopped at many sites working our way north like the Nebraska City Interpretive center and Sgt. Floyd Monument. Camped by the water at L&C State Park in Onawa, Iowa, and at American Creek RV in Chamberlain, SD by the Missouri. Several of the Info Centers had small museums with keelboat replicas and statues, always with Seaman, the dog. Climbed up Pompey's Pillar in Montana to see Clark's signature carved in the rock.
This summer, we saw the Great Falls Interpretive Center(with its resident Newfoundland dog) and, on the Pacific Coast, camped at Fort Stevens State Park to visit the L&C National Historical Park. There are still lots of points we haven't yet seen, but it's an enjoyable quest.