I thoroughly enjoyed Once a Warrior King by David Donovan. He was an advisor in Vietnam, a fascinating glimpse into a little known side of the war. He was part of a MACV team leading local militia defending their own village. He became the title of the book.
My other favourite is Destroyer Captain by Roger Hill. He commanded a destroyer almost continously from 1942 to 1945, famously for being one of the ships strapped to the Ohio to bring it in to Malta. What struck me was the honesty with which he detailed how the strain of wartime command affected him.