There's No Place Like Home by Cecilia Ahern

Cecelia Ahern, author of P.S., I Love You, writes a whimsical story about an Irish girl named Sandy Shortt who becomes obsessed with finding lost things after a neighborhood girl goes missing. Sandy's parents send her to a psychologist to no avail. When Sandy grows up she moves to Dublin to track down missing persons for a living. A man asks her to find his missing brother so he can be at peace knowing what has become of him. Sandy gladly takes the case but soon after, she goes out jogging and finds herself in a parallel universe with all types of missing people and things. To her horror, it appears she can never return to her original life.