At an early age, he moved with his parents to the home of his maternal grandmother at Valley Forge and the tradition of George Washington's winter encampment there during the American Revolutionary War fueled a lifelong interest in military matters.

Zook entered the emerging field of telegraphy, became a proficient operator, and worked on crews to string wires as far west as the Mississippi River.

Zook was personally scouting far out in front of his regiment in the run-up to the Battle of Gaines' Mill, got behind enemy lines, and found that Confederate Maj. Gen. John B. Magruder was conducting an elaborate deception, making it appear that he had significantly more troops in his sector than he actually had.

The brigade was one of the first to arrive at Fredericksburg, Virginia, and he wanted to cross over the Rappahannock River as quickly as possible, before Confederate General Robert E. Lee could reinforce the town and the heights beyond it.

Zook had his horse shot out from under him and was momentarily stunned, but managed to lead his men to within 60 yards of the Stone Wall, one of the farthest Union advances of the battle.

Despite his successful promotion, however, the Battle of Fredericksburg affected him deeply: I walked over the field, close under the enemy's picket line, last night about 3 o'clock.

To see men dashed to pieces by shot & torn into shreds by shells during the heat and crash of battle is bad enough God knows, but to walk alone amongst slaughtered brave in the "still small hours" of the night would make the bravest man living "blue".

God grant I may never have to repeat my last night's experience.At the Battle of Chancellorsville in May 1863, Zook's brigade fought in the defensive line around the Chancellor Mansion, but facing east, where combat was lighter and his men suffered only 188 casualties.

Zook was directed by one of the III Corps staff officers toward the Wheatfield to reinforce the brigade of Col. Régis de Trobriand and to fill a gap near the Stony Hill.