Mark Twain's Life on the Mississippi is both a memoir and travel book described as "a great grab-bag of a book" that recalls Twain's youth when he learned to navigate the mighty Mississippi and later became a steamboat pilot on a river that changes constantly so that charts are of little use. One criticism of this book is that there is no unification of direction in it as Twain recounts his days as an apprentice, then as a steamboat captain. In the latter part, the book is more of a travel document as an older Twain returns to the river that he so loves, and he catalogs the changes to the shores and surrounding areas since cities have sprung up along the river. 

So, perhaps the thesis is a sentence in the second paragraph of the first chapter: "It is a remarkable river." Since the thesis is the unifying idea, only a sentence about the Mississippi River can be the idea that gives any unity to this book. Further, after this sentence there are the beginning sentences of subsequent paragraphs that reiterate and reinforce the remarkableness of the river: 

The Mississippi is remarkable in another way....
Unquestionably the discovery of the Mississippi....
The river's earliest commerce.....
I went to work now on how to navigate the river....