On this subject I'm currently reading the following:
"What Hath God Wroght: The Transformation of America, 1815-1848" by Daniel Walker Howe, 3rd book in the Oxford history of the United States.
Seems very well documented and so far a pleasure reading.
On the shelves but haven't opened yet are:
"A Country of Vast Designs: James K. Polk, the Mexican War and the Conquest of the American Continent" by Robert W. Merry.
"Lions of the West: Heroes and Villians of the Westward Expansion" by Robert Morgan.
Hope this helps.