The Messiah College Brethren in Christ Historical Library and Archives contains primary and secondary sources for those with Brethren in Christ or "River Brethren" ancestors. The home page for the Archives provides a synopsis of their holdings with primary sources, special collections, books and periodicals. Information about the Archive and their guidelines can be found here. Looking for a specific Brethren member? This list of available manuscript collection authors includes church leaders and lay members.
Because there are various Brethren groups, you may want to contact the Archives first to make sure your Brethren ancestors would be included in this collection. A list of Brethren groups is here.