Sometimes you can tell a book by its cover – at least this kind of cover. The devilishly handsome man with the unbuttoned shirt and manly biceps … the comely woman a-swoon in his arms, her dress cut perilously low  … it’s the bodice-ripper, the romance novel. Sometimes it’s a very explicit romance novel. “Fifty Shades of Grey” has upped the ante in the genre, but there’s also what you might call “Fifty Shades of … Khaki.” Military romantic erotica is about men and women in and out of uniform, and Patt Morrison speaks with Cat Johnson, one of the authors in the anthology called “Duty and Desire” about the very particular audience for this fiction.