The letters are by Lt. Edward Granger. Granger was commissioned as an officer in the 5th Michigan who joined the staff of Custer in 1863. Granger wrote very insightful, detailed letters, and they're really good. His description of the Battle of Yellow Tavern is the best one I have ever seen. Granger was KIA at the Battle of Front Royal in August 1864.
These letters are not in any repository, and until they were compiled by my collaborator, they have not been seen outside the family. When they are published, it will be the first time that they will be available to the public. That's part of what I like about the project.