Tomorrow and Tomorrow and Tomorrow by Gabrielle Zevin (HB)

This is the story of Sam and Sadie. It's not a romance, but it is about love.
When Sam catches sight of Sadie at a crowded train station one morning he is catapulted straight back to childhood, and the hours they spent immersed in playing games.
Their spark is instantly reignited and sets off a creative collaboration that will make them superstars. It is the 90s, and anything is possible.