While Powers served from Sept. 9, 1864, through March 29, 1865, as captain of Company H, 47th Missouri Infantry, his "historical significance" lies primarily in regard to his counter-insurgency operations against Confederate irregulars while serving as captain of Company A, Wayne County Cavalry (aka Haw Eater Scouts) from Oct. 12, 1861 through Feb. 20, 1862; captain of Company K, 68th Enrolled Missouri from July 28, 1862 through March 26, 1863; and captain of Company M, 8th Provisional Enrolled Missouri Militia from May 12, 1863 through October 31, 1863.
While it doesn't go into a whole lot of detail, my book "The Autobiography of Samuel S. Hildebrand, the Renowned Missouri Bushwhacker," published in 2005 by the University of Arkansas Press, is one of the more comprehensive published sources of material on Pinkney Powers and the units he served in. A quick check of the index indicates references on at least 8 pages to Powers, 2 pages to the Haw Eater Scouts, 17 pages to the 68th EMM, 6 pages to the 8th PEMM, and 5 pages to the 47th Missouri Infantry.
Quoting from a letter written by H.C. Wilkinson (who served directly under Powers in all of the above-referenced units except the Haw Eater Scouts), here is a sampling of what my Hildebrand book says on Powers (from pp. 203-204)--
"Very soon after arming of Co. M at Mineral Point, somehow Capt. Powers 'scented game' above. Sam Hildebrand and his band was somewhere up country and Capt. Powers, true to his instinct as a scout, obtained permit to take with him picked men, and by night marches, reached his home in Wayne County and steal over onto Castor River in the night to watch for Hildebrand to come along on his way south. Co. M armed with the Enfield Carbine, and several revolvers, they left camp at Mineral Point in Washington County late p.m. The 3rd night out found them posted on the Bloomfield and Fredericktown Road on Castor River, in the barn shed on the farm of Solomon Whitener and some 3 miles south of Gravelton. They had hardly settled to business when the guard in obedience to orders stole in, and reported horsemen approaching. Soon they came along side and one made a pass to shoot Capt. Powers' guard, but, all at once 3 out of the 4 Rebels lay in the road dead men, one of the 4 escaped but cut all over by bullets. Capt. Powers had 17 trusty men with him and their fire proved it. Daylight revealed the fact that Sam Hildebrand was about a hundred yards or more to the rear with some 8 or 10 men with him, but at the fire of Capt. Powers and his men on his advance gaurd they took to the hills leaving the road. This affair put an end to guerrilla bands traveling that road for a long time."
While personal accounts of veterans of the Civil War are notorious for being exaggarated and/or manufactured, Sam Hildebrand himself stated that he and his men came out of this scrap "somewhat worsted," a significant admission from a man who was prone to exaggeration himself and rarely ever admitted to having been defeated.