Missive starts with the familiar my-grubby-apartment setting, but really it's about a murder mystery wrapped in word puzzles - armchair detective work at its finest. An alternative headline for this would naturally be "a riddle wrapped in a mystery inside an enigma."
The puzzles are optional, not connected, and of the cryptic crossword type. A phrase in the text might prompt the reader to look for, say, every third letter of each word. These were pretty fun, even if most of the puzzles were completely unintelligible to me.
Good if you like cryptic crosswords and lots of wordplay loosely connected to plot.