I just finished a translated copy of EXPLORING WITH FREMONT. It was written in German by a gifted young cartographer (in diary form) by the name of Charles Karl Preuss. Preuss was employed by John Charles Fremont on his 1st, 2nd, and 4th expeditions into the Oregon Territory and areas covering California, New Mexico, Colorado, Wyoming, Nevada, Utah and almost all of the area west of the Missouri River. Preuss was able to finally produce an accurate map of the newly discovered Oregon and California trails, the correctly proportioned relationship of all the rivers, mountain ranges, deserts and passes......no more 'educated guesses' on the maps.
Most of these old diaries can be pretty boring, but I must say, this one is the most entertaining of any that I've read. Preuss constantly gripes about the food and camping conditions.

Did you know that the head is the tastiest portion of a dead mule?

His opinion of Fremont is pretty telling, too. He complains about the stupid moves and quirks that Fremont displays in his everyday decision making, etc.
Guides, Kit Carson, Jim Bridger, and Old Bill Williams are discussed at length.....very interesting highlights into each man's skills and weaknesses.
I had to get it through inter-library loan, but if I ever get a chance to buy a reasonably priced copy, I WILL own one.

Portions of Preuss' mapping efforts are available for viewing at bottom of this page..
http://www.longcamp.com/preuss.html