A Trace of Memory by Keith Laumer

The two-hundred pound señorita with the wart on her upper lip put a pot of black Cuban coffee and a pitcher of salted milk down beside the two chipped cups, leered at me in a way that might have been appealing thirty years before, and waddled back to the kitchen. I poured a cup, gulped half of it, and shuddered. In the street outside the cafe a guitar cried _Estrellita_.

"Okay, Foster," I said. "Here's what I've got: The first half of the book is in pot-hooks -- I can't read that. But this middle section: the part coded in regular letters -- it's actually encrypted English. It's a sort of résumé of what happened. " I picked up the sheets of paper on which I had transcribed my deciphering of the coded section of the book, using the key that had been micro-engraved in the fake scratch on the back cover.