Manuel has authored four books: Andrea Costa and the Rise of Socialism in the Romagna (1980), The Hispanic Elite of the Southwest (1989), The Politics of Fear (2006), with Richard Delgado, and Mexicanos: A History of Mexicans in the United States (expanded edition in 2009). He is co-author, with his wife Cindy, of a fifth volume, En Aquel Entonces: Readings in Mexican-American History (2000). He has written many book reviews and articles for professional journals, as well as encyclopedia entries. He has contributed regularly to the DVC faculty newsletter, the Faculty Senate Forum.
Manuel has been teaching at DVC since 1971. His history courses focus on the United States, Latin America, and Europe. He has been a visiting professor in the Ethnic Studies department at UC Berkeley (1993) and is currently an adjunct professor in the History department at CSU East Bay. He taught at DVC study abroad centers in London (1992) and Florence (1995). Manuel was faculty advisor for three student clubs: La Raza Unida, Alpha Gamma Sigma, and the DVC Book Club. He served on the advisory boards of several campus programs, including EOPS/CARE; on ten Social Science division hiring committees; and as NEXUS mentor for five colleagues. He was selected by the DVC Faculty Senate as campus candidate for the Hayward Award for Excellence in Teaching in 2000 and as the college Faculty Lecturer in 2007. He was co-chair of the accreditation self-study committee on faculty and staff in 2003.
Manuel attended Coalinga Junior College (now West Hills College), receiving an A.A. degree in 1962, and life membership in Alpha Gamma Sigma. He transferred to UC Santa Barbara, where he earned a B.A. with high honors in 1965, an M.A. in 1967, and a Ph.D. in modern Italian history in 1972.
Manuel has spoken on a variety of historical topics at high schools, colleges, bookstores, and civic groups. He also served many years on the executive board and the scholarship selection committees of the Kennedy-King Foundation.
Manuel is a member of the California Historical Society, the Irish Literary and Historical Society, the National Association for Chicana and Chicano Studies, the Society for Italian Historical Studies, and the Western History Association.