It was an interesting book, but essentially the re-telling of personal failure in epic terms. Lewis was a very bright and troubled man. Because he's too depressed and overwhelmed by responsibility to organize and publish his journals, much of what actually occurred on the voyage of discovery remained unknown for a century after it was undertaken.
It's been a while since I read the book, but I believe Ambrose makes a point of mentioning that none of the rivers, mountains, or western geopgrapic features bear names given them by the expedition. They bear the names of later arrivals because Lewis was too despondent publish his records.
The expedition became a teaser of sorts to easterners, but revealed very little information about the west to an interested population.Basically the people who anted the information the expedition was undertaken to supply, were left unsatisfied. Imagine the moonlanding untelevised, and unreported. Imagine our impression of the moonlanding if it depended on the records of Neil Armstrong.
It's a good read, but one can't escape the impression that if the expedition had never been undertaken, it wouldn't have made much difference. The difference that was made, was wrought by other people who never read or were even aware of the journals. For all intents and purposes it might just as well never have occured.