You can read James K. Polk's inaugural address from March 4th, 1845, available through the Avalon Project at Yale University.

The failed Treaty for Annexing Texas, submitted to the Senate on April 22nd, 1844 and subsequently rejected on June 8th by a vote of 16 "ayes" to 35 "noes."

The Congressional Joint Resolution admitting Texas into the United States, December 29th, 1845.

President Polk's May 11th, 1846 message to a joint session of Congress on the state of relations between the United States and Mexico.

You can read Henry David Thoreau's justification for his refusal to pay his taxes and his desire not "to be associated with Massachusetts, either in holding slaves or in conquering Mexico," in the online version of his work, A Week on the Concord and Merrimack Rivers.

The Treaty of Guadalupe Hidalgo from February 2nd, 1848 is available online.

Army military historians have chronicled the dramatic events surrounding the American occupation of Mexico City, with some valuable maps and contextualization of the situation in Mexico when hostilities began.