When John Leganski walked into Kevin McCarthy’s office as an intern in 2012, he’d barely heard of the man who would come to define his political career. But now, Leganski’s something of an expert — so much so that he could write a book about the former speaker. Which is exactly what he did. “Glory, Grief and the Gavel” details what it took to elect the California Republican as speaker after 15 rounds of voting and the fight that ultimately le…