Every universe has its jargon, and for book critics, one of the most commonly used — and loaded — descriptors is “experimental.” All too often, it’s code for difficult, or self-conscious, or overly writerly. It’s a problem, then, when a book comes along that’s both innovative and a genuine pleasure, both impressively clever and just plain absorbing. Dept. of Speculation is, by all means, a formally adventurous novel, choosing to tell its story via vignettes and letters and tangents and scattered scenes that, yes, can take some thinking to parse; its main characters, a husband and a wife, aren’t even given names. But author Jenny Offill, who’ll read at Diesel (5433 College Ave., Oakland), is plainly talented, and just when the book feels like it’s too messy, too much, she brings you back to earth, engrossed all over again. Sometimes experiments yield miraculous results.