The book is twenty-one letters. I use the word "letter" loosely. A surgical dictation is a letter. A cockpit voice recorder transcript is a letter. A recipe card annotated by three generations of the same family is a letter. A homestead deed from 1884 is a letter. A radio signal broadcasting Chopin and a list of forty-seven names into a dead frequency is a letter. A mathematical theorem inscribed into the DNA of a bacterium is a letter. Each one crosses a gap. The first gap is one second. A surgeon dictating an operative report while the patient is still on the table. The last gap is 4.24 light-years. That is the distance to Proxima Centauri, the nearest star, and it is also zero, depending on your frame of reference. Special relativity tells us that a photon traveling at the speed of light experiences no transit time. From the photon's perspective, emission and absorption are the same moment. The gap is a property of the receiver, not the sender. The message does not know it is late. Between those two extremes, one second and light-years, I tried to cover every register of human communication I could reach. The clinical and the domestic. The bureaucratic and the intimate. The comic and the elegiac. The personal and the geological. The living and the dead.