I've written lengthy blog posts on these two books twice and the computer crashed before they were saved, so let me try to keep it short this time.
Kernan joined the carrier Enterprise at Pearl Harbor a few days before the attack on Pearl Harbor and was there as an enlisted man working on squadron maintenance through the Battle of Midway. That means that he was on the flight deck of Enterprise when the torpedo planes came back mauled, the fighters came back having never engaged the Japanese, then when the dive bombers came back to report three burning Japanese carriers.
He then transferred to the Hornet and was on her till she sank at the Battle of Santa Cruz.
He then became a rear seat gunner and returned to the Enterprise in 1943. He was looking down at Butch O'Hare's plane from his position as rear gunner in a TBF on night combat patrol when O'Hare was shot down. If you believe O'Hare was shot down by friendly fire, it was McKernan that did it, although he explains that that isn't what happened – he saw O'Hare flip on his cockpit lights for a second, and then a Japanese planes moved in behind him and shot him down. He did fire shortly before that, but not at O'Hare, he believes.
After that McKernan went into flight training to be a pilot, but given the long line of pilots waiting for assignments, he (and many others) gave up waiting and went back to rear seat work on escort carriers.
After the war McKernan eventually became a classics professor at Yale. In 1994 he wrote Crossing the Line, which was published by the Naval Institute Press, and is quite simply the best firsthand memoir of the Pacific War I have ever read. As you might expect from his academic credentials it provides a well-written story of a dirt-poor Depression era boy's experiences as an enlisted man in the Navy, from combat to shipboard life to what life when off the ship was like. His Forrest Gump-like experiences at Pearl Harbor, Midway, Santa Cruz and with the first night fighter squadrons are just incredible to read.
In 2004 McKernan's memoirs were republished by Yale as part of their Library of Military History series, and the next year he added an analysis of the U.S. torpedo squadrons in The Untold Battle of Midway: The Destruction of the American Torpedo Squadrons, in which he added his firsthand experiences on the Enterprise that day to the by-then extensive literature on the battle, as well as an academic's penchant for research and analysis.
This second book is particularly valuable because of when it came out as well. By 2005, there had been numerous well-researched accounts of the battle that had revised the original stories about what happened based on newly analyzed information, including earlier that year Parshall and Tully's Shattered Sword (cited in McKernan's bibliography) which extended these revisions to the Japanese side of the battle. Although McKernan's book ostensibly deals with the torpedo squadrons, it actually includes an extensive analysis of the tactical conduct of the battle using all this later scholarship about, for example, why the Yorktown was so effective on June 4, and the Hornet - and to a significant degree Enterprise, was not.
Earlier accounts of the American side of the battles were hamstrung by not having the then-classified information reflecting the US carriers' tactical plan for the day (Enterprise and Hornet were cocked and locked to attack as soon as the Japanese carriers were sighted, with Yorktown deliberately held in reserve – that was not known for at least half a century), as well as by the almost complete dearth of action report-type information from the Hornet's squadrons. It took decades for historians like McKernan to reconstruct what actually happened to Torpedo 8 and and Bombing and Scouting 8, but by 2005 the pieces were available to tell the story that Hornet's captain Marc Mitscher and particularly its air group commander Stanhope Ring apparently did not want told.
So while Crossing is an invaluable memoir, Unknown is a useful addition to the Midway corpus, and actually a good short study of the tactics on the US side.
Occasionally we are fortunate to have someone there when history is made who is observant and able to feed their experiences directly into the historical record. McKernan is the best example of this I have yet encountered.