The Congregational Christian Historic Society has over 255,000 volumes relating to the protestant faith of Congregationalism which includes the United Church of Christ. There's a lot to like about this website including a necrology search on the homepage with obituaries for 25,000 clergy and missionaries covering more than two centuries.
An Online Catalog allows you to search their collections. A Resources tab includes a history of the denomination, resources including primary sources and a tab for archives and finding aids. Don't forget to check out the Digitized Resources under the Resources tab for periodicals, digitized manuscripts and more.
This Library is on Facebook and Twitter. So there are lots of ways to learn about their collections. Make sure to join their mailing list to keep up to date with their collections.