In doing a quick search of the Official Records in Series 1, Vol 41, part 1, which covers the period from July 1, to Dec 31, 1864, list 46 references to Monticello, Arkansas, during that time period.

Of most interest probably to you would be the report of the Expedition from Pine Bluff to Monticello Ar. September 9th, 10th, and 11th, 1864, particularly the skirmish at Monticello, September 10th, 1864, reported on page 758.