Memoir
And here, Shmoopers, is where we talk about the difference between autobiography and memoir. Sometimes people use those words interchangeably, which is understandable—they are both stories of someone's life—but the key difference is the timeline. Autobiography is a chronological narrative of the author's whole life, while memoir focuses on the events of a specific, and usually short, period of time.
So yes, Pilgrim at Tinker Creek is an autobiography, in that it's a true story from Annie Dillard's life, as told by Annie Dillard. However, it's more accurate to call it a memoir, because it focuses on one long, lonesome, bug-filled year. Sure, she talks about childhood a little, like when she tells the story about hiding pennies at the beginning of Chapter 2. But that story, and the other flashbacks (for instance, to the flood that occurred the year prior) illustrate something Dillard wants to tell us about the present—by which we mean, of course, 1974.