I don't have secondary sources to offer, but I have been working with a lot of primary sources lately, particularly correspondence, from exactly this time period.

The consensus, at least among liberal folks in the Midwest, was that Nixon was corrupt and used his position of power unfairly for his own benefit. Ford pardoning Nixon meant that Nixon wouldn't be held accountable for his actions. It also meant that both Ford and Nixon were continuing to use their positions of power to protect the powerful. An ordinary American wouldn't get pardoned by the president for what Nixon had done. Ford later admitted that his main motivation for pardoning Nixon was their friendship. The exceptionalism and anti-egalitarianism implied in the pardon rubbed people the wrong way and soured a lot of Americans to Ford.

It seemed to represent a second betrayal and abuse of power. In their eyes, it tainted the new administration by pardoning the corruption of the first. There was also a semi-popular belief that Nixon only agreed to resign and pass the presidency to Ford if Ford agreed to pardon him.

Letters from the weeks and months following the pardon are cynical about the administration, not necessarily because of Ford but because of the choice to pardon Nixon. I remember one letter in particular from an American novelist who claims he quite liked Ford and was hoping the new administration would prove a fresh start for America, but was completely disenchanted after the pardon.

You can find lots of other evidence of the controversy of the decision, including newspaper articles and various officials resigning in protest. Seeing the opinions of normal Americans scribbled on notepads with the other news of their lives, jobs and visits and deaths, really put the event in perspective for me, and made me realize why so many people felt betrayed.

A.B. Guthrie, Jr.'s correspondence at Yale University is particularly fascinating if you're interested in the evolution of one man's political opinions over the course of the late 20th century. Once a staunch Republican, Guthire became dispirited with Nixon, disenchanted with Ford, and absolutely infuriated with Reagan. He has an absolutely razor-sharp wit, really distinctive turn of phrase, and writes about politics freely and often. By the end of his life, he was in an open feud with his very Republican son, and claimed that he could never again vote for a Republican. Seeing the effects of national politics on the life and writings of one man in rural Montana was a truly fascinating experience.