By the time the ACW came around the militia system had become a good deal more organized than 100 years before. The individual states would generally have some degree of say over the organization and the grouping of units from individual communities into regiments.

When Lincoln first put out the call for 75,000 men from the states to put down the rebellion this was in a large part filled by either 90 day regiments, or at most 1 year regiments formed from a combination of pre-war units and an influx of volunteers.

The way the system worked was that the US government would not take a regiment into service unless it was fully equipped and ready, so it was on the states to handle it themselves, then apply to the War Dept to accept the unit and give them orders. These units, which made up the bulk of the Union Army were known as United States Volunteers, and would eventually constitute 3 year regiments, and men enlisted for the duration of the war.

There were also units that were retained in their home states and did not go into Federal service, or only did so at a later time. The Pennsylvania Reserves would be an example of this, though they later went on to give a good account of themselves in field service.

In the South the situation was even more complex. With states like Georgia in particular who were very jealous of their regiments and attempted to keep as many in state service as they could, though in the end it did them little good and drained men away from the field armies.

In the end a system like found in the Revolution would just not have been practical. The small regular army was just not able to serve as some sort of core where local militia could rally around. The scale of the fighting was far to large and the distances too great.

Would highly recommend anything by Bruce Catton or James McPherson.

I know as a part of local history, Pennsylvania and New York militias were involved in the Gettysburg Campaign. For instance, they delayed Stuart's cavalry at the Battle of Carlisle.