Frederick H. Dyer, July 2, 1849 - September 21, 1917, enlisted in the Union Army at the age of 14, serving as a Drummer Boy. After the war Dyer spent nearly 35 years collecting information and another 5 years compiling that information for publication. His work documented the regiments and their actions during the War of the Rebellion, which he self published under the title "A Compendium of the War of the Rebellion".
The Compendium is primarily used to understand the units by which the army was structured and the individual battles and actions in which they participated. If you know the State and Regiment to which a soldier was assigned, as well as the dates of service, it is relatively easy to determine where they may have been involved in combat, keeping in mind that simply because a Regiment saw combat in a particular battle, an individual soldier assigned to that Regiment may not have been present, or may not have been utilized.