IF THERE’S ANYTHING poet Jack Spicer (1925–1965) and his archivist-biographer, the late Kevin Killian, can teach a reader, it’s that anything can be a poem, even a letter. The letter, as a literary thing, is one of several “occasional” forms, like “Dedication to Your Swimming Pool” by the Roman poet Statius, written to crash a rich man’s pool party, drink his wine, and feel up his spouse. It’s occasional like lectures, introductions, obituaries, sermons, thank-you notes, dedications, dirges, odes.