“Crooked Letter, Crooked Letter,” by Tom Franklin (William Morrow, 288 pp.) is a page-turning mystery with a sentimental heart. As boys Silas and Larry lead very different lives and a few months of secret friendship is their only commonality. Years later, as adults, they are reluctantly brought together over an unsolved crime, forced to confront the lingering suspicion aimed at Larry. Can the same incident that drove them apart revive their friendship?