One of the great resources at my disposal while researching and writing the book was the Holy Cross History Association.
According to their website (here), the mission of the Association is to:
"...promote and stimulate historical study of and research on those religious communities which trace their origins to the Rev. Basil Moreau of LeMans, France; to discover, collect and preserve historical manuscripts; to print, publish and cause to be distributed, papers, books, writings, reports, articles and data bearing on or in any way relating to the Congregations of Holy Cross."
The Association accomplishes the goal by publishing a newsletter and sponsoring annual conferences (information about the 2011 conference can be found here).
Among their very best resources is the availability of the presentations given at the annual conferences at a very nominal cost (only $1 plus postage!).
A list of the papers is here and members also receive an excellent guide to the papers by person, place, or subject.
Just a few of the papers I found helpful were:
"Holy Cross Communities in the Civil War" by Rev. James T. Connelly, CSC, 16pp. (1993)
"Grace under Pressure: Sisters of the Holy Cross, 1861-1865" by Ms. Barbra Wall (2000)
"Holy Cross Military Chaplains in World War II" by Rev. Joseph A. Kehoe, CSC, 19pp (1995)
Even if you are not writing about the Holy Cross order, Notre Dame, or specific personalities (priests, brothers, sisters, etc.) per se, do consider browsing the available papers because they might enrich other research you are doing, including regional studies, as the mission of the Congregation has since extended well beyond the campus of Notre Dame to include cities and institutions all across the country over the past almost 170 years.
Other resources on the website include an excellent Bibliography (PDF here) of over 100 pages and many hundreds of citations.
Membership is only $5 (here) and I have happily been a member for the past couple of years and will continue to support the Association as a member.
Many thanks to them for all that they do!