American travel writer Maggie Shipstead's third novel Great Circle, is about a fictional aviatrix, Marian Graves, who goes missing in her quest to circumnavigate the globe in 1950. It's also about a troubled Hollywood actor who attempts to resurrect her reputation by taking the lead role in a Marian Graves biopic. The novel explores questions of death and disappearance, as well as the unknowability of the past. Maggie Shipstead speaks to Sarah L'Estrange.