The Time Traveler’s Wife by Audrey Niffenegger

Clare, a beautiful art student, and Henry, an adventuresome librarian, have known each other since Clare was six and Henry was thirty-six, and were married when Clare was twenty-three and Henry thirty-one. Impossible but true, because Henry is one of the first people diagnosed with Chrono-Displacement Disorder: periodically his genetic clock resets and he finds himself misplaced in time, pulled to moments of emotional gravity in his life, past and future. His disappearances are spontaneous, his experiences unpredictable, alternately harrowing and amusing.