I think the book you want is Traveling with the Innocents Abroad, edited by 
Daniel Morley McKeithan. It contains all 50 letters from the trip to Europe 
and the Holy Land -- all the ones that appeared in the Alta, plus a handful 
of others written for a New York paper.

Travels with Mr. Brown has 25 letters from the time Twain left San Francisco 
until the Quaker City set out from New York. It includes a side trip he made 
to Missouri while he was waiting for the tour to begin. As I recall, parts 
of the last letter or two found their way into Innocents Abroad, but the 
rest of the book has not been collected anywhere else.

-- Bob G.