Charlotte Sometimes.

Charlotte Sometimes was a children's book written by Penelope Farmer, published in 1969. It is the third and best known of three books featuring the Makepeace sisters, Charlotte and Emma, and inspired the song "Charlotte Sometimes"by English rock band The Cure. 

Summary:

Part one:
Charlotte arrives at a new boarding school, and is shown around by a prefect named Sarah. Sarah's mother also attended the school. The next morning she finds herself in the same place, but in the year 1918. A younger girl called Emily calls Charlotte her sister, and addresses her as "Clare". Each night, Charlotte finds herself swapping between her own time and Clare's time. Charlotte and Clare must learn to live two different lives. They write letters to one another in an exercise book, which they hide in the leg of the bed they share in two different times.

Part two

Charlotte, expecting to have returned to her own time for the last time, is shocked to find that she has returned to 1918. She will go into lodgings with the Chisel Brown family: it appears she will be trapped in the past. In the house, Miss Agnes Chisel Brown shows Charlotte and Emily the toys she had once played with. She tells the two girls about her brother Arthur, who died in the war. Charlotte reflects, forward and back: to Arthur in the past; her own sister Emma in the future; and Clare, trapped in Charlotte's time. She struggles with her identity as being Charlotte sometimes but Clare other times.
Charlotte and Emily form a plan to enter the school by night in an attempt to get Charlotte into the bed which will take Charlotte back to her own time. Inside the school sick room, Charlotte finds the bed is occupied, and thus she cannot return home. She escapes being seen by Nurse Gregory, but is seen by another student, Ruth.
Charlotte is not the only one who struggles with identity. Emily tells of the wretchedness of being motherless and unwanted, moving around between homes while her father fights in the war. Meanwhile, Charlotte dreams she is fighting to stay as Charlotte. She dreams about Arthur.
A letter arrives for Clare and Emily from their father. Emily does not let Charlotte read it, to the bewilderment of the other girls. Charlotte, thoughtful as always, wonders who Sarah's mother is: perhaps it will be Charlotte herself if she is trapped in 1918?
At night, Charlotte dreams about Arthur again, as a drummer boy, and that she has turned into Agnes. Her crisis of identity comes to a head as she struggles to preserve her identity as Charlotte.
One evening, the Chisel Browns hold a seance in an attempt to speak to Arthur. The girls hide behind the curtains to observe. During the seance, they hear Clare's voice crying out for Emily. Emily cries out, and the two girls are discovered and disciplined. Later, Miss Agnes asks about the voice they heard at the seance - Clare's. She then tells Charlotte and Emily of Arthur’s war experience.
Finally, Armistice comes. The war is over: people dance and celebrate in the street, and Charlotte and Emily join in, even though it would anger Mr Chisel Brown. In disgrace, Charlotte and Emily are sent back to the school. Miss Agnes gives them the toys as a gift.

Part three

Ruth recalls her “dream” of seeing Clare whilst in the sick room. Because of the flu epidemic, the students are able to play wild games in the dormitories, and eventually, Charlotte is finally able to sleep in the bed that will return her to her own time.
On arriving back in her own time, Charlotte is surprised to learn that her room-mate Elizabeth knew about her swap with Clare. Charlotte wonders about Sarah's mother and what has become of Emily and Clare. At the school, Charlotte sees an elderly Miss Wilkin, whom she realises that she had known as a young woman in 1918.
One day, Charlotte has a conversation with Sarah, and learns what has become of Emily and Clare. Sarah's mother is Emily and Clare died in the flu epidemic after the war. Later, Charlotte and Elizabeth discuss the events Charlotte has experienced. They find the exercise book in the bed leg, finding the last letter Charlotte wrote to Clare.
Charlotte receives a package from Emily as an adult. It contains a letter from Emily and the toys which Miss Agnes had given them, over forty years ago. I consider this book a classic. I highly recommend it to anyone who enjoys a god old adventure book! :)