There are several regimental histories/collections of letters on the 42nd OVI, but the book on Garfield I found particularly interesting is The Wild Life of the Army: The Civil War Letters of James A. Garfield, Frederick D. Williams, ed. (Michigan State University Press, 1964).
There are only a couple book length biographies of Garfield, and they treat his service at length. Margaret Leech, "The Garfield Orbit" is a classic. "Destiny of the Republic" by Candice Millard is newer and may be easier to find. Garfield's presence is also large in ""Blood, Tears and Glory." I'm very fond of Garfield and feel his presidency is one of American history's great what-ifs.