The author published a peer-review article with sources:
Atheism, Secularity, and Well-Being: How the Findings of Social Science Counter Negative Stereotypes and Assumptions, Phil Zuckerman, Sociology Compass 3/6 (2009): 949–971, 10.1111/j.1751-9020.2009.00247.x
But more significantly, when we actually compare the values and beliefs of atheists and secular people to those of religious people, the former are markedly less nationalistic, less prejudiced, less anti-Semitic, less racist, less dogmatic, less ethnocentric, less close-minded, and less authoritarian (Greeley and Hout 2006; Sider 2005; Altemeyer 2003, 2009; Jackson and Hunsberger 1999; Wulff 1991; Altemeyer and Hunsberger 1992, 1997; Beit-Hallahmi 2007; Beit-Hallahmi and Argyle 1997; Batson et al. 1993; Argyle 2000).
Here's an analysis of the sources I could read, unfortunately some of them are books which I can't access at the moment. I don't know to what level the books would support the author's point, however the article has been peer reviewed, the books seem scholarly, well reviewed and in-depth. See Psychology of Religion: Classic and Contemporary: Classic and Contemporary Views as an example of such a book.
Studies are reported of university students, and of their parents, that found that religious fundamentalism correlated quite highly with religious ethnocentrism, as well as with-to lesser degrees-hostility toward homosexuals and prejudice against various racial-ethnic minorities. Also, fundamentalist students reported receiving strong training in identifying with the family religion from an early age. But, by comparison, they reported virtually no stress being placed on their racial identification. It is suggested that strong, early emphasis of the family religion may reinforce Tajfel's minimal group effect and produce a template for "us-them" discriminations that facilitates acquiring later prejudices.
Bob Altemeyer, RESEARCH: Why Do Religious Fundamentalists Tend to be Prejudiced?, International Journal for the Psychology of Religion, Volume 13, Issue 1, 2003.
Apparently, religious fundamentalism and nonquesting are linked with authoritarianism and prejudice toward a wide variety of minority groups. Possible explanations for these relationships are discussed.
Bob Altemeyer & Bruce Hunsberger, Authoritarianism, Religious Fundamentalism, Quest, and Prejudice, International Journal for the Psychology of Religion, Volume 2, Issue 2, 1992
In a classical study of ethnocentrism and antiSemitism,
the scores of those respondents reporting no religious affiliation
were found to be lower than members of most religious groups (Adorno et
al. 1950). The authors summarized the findings as follows: 'it appears that
those who reject religion have less ethnocentrism than those who seem to
accept it' (1950, p. 213). These findings have been confirmed many times
Benjamin Beit-Hallahmi and Michael Argyle, The psychology of religious behaviour, belief and experience, Routledge, 1997