"The Time Traveler's Wife" by Audrey Niffenegger

Clare and Henry have known each other since Clare was six and Henry was 36. They were married when Clare was 23 and Henry was 31. 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 from his life, past and future. His disappearances are spontaneous, his experiences unpredictable, alternately harrowing and amusing.

Clare and Henry's story unfolds from both points of view, depicting the effects of time travel on their marriage and their passionate love for each other. They attempt to live normal lives, pursuing familiar goals: steady jobs, good friends, children of their own. All of this is threatened by something they can neither prevent nor control, making their story intensely moving and entirely unforgettable.