'Jill' by Philip Larkin is an insight into the stratified world of 1940s Oxford University. A young man, John Kemp from the north of England ends up sharing a room at the University with a brash, uber-confident Londoner, Christopher Warner. Kemp's desperation in trying to get with the in crowd even as they mock and denigrate him at every turn reveals some incredible class differences. The reader feels constantly embarassed for John Kemp as the more he gets mocked, the more it seems to sharpen his desire to be part of their circle.
A turning point in the story comes when John feels he may have discovered a possible chink in Christopher's seemingly impenetrable armour. Could it be that Christopher may envy him his warm family ties? This discovery thrills John so much that he ends up inventing a whole fictional narrative around a loving younger sister named 'Jill' he doesn't actually have.
This harmless pursuit initially involves leaving invented letters from Jill lying around their shared room in the hope that Christopher will secretly read them. John diligent creation of this world of make-believe turns him into something of a writer of fiction. Through this he is hoping all the time that Christopher will take a real interest in his life and start noticing him more, unlike his current interest levels where John may as well be part of the furniture in their room.
For John this is all innocuous stuff till one day his 'character' Jill actually materialises in Oxford in the form of a teenage girl coincidentally called Gillian. John becomes instantly infatuated with the real Gillian, fuelled no less by his own creation of her. He pursues her madly; following her on buses, waiting for her in cafes and showing up at parties where she is likely to be.
Read the book to find out how John Kemp's pursuit eventually ends.
PS: Philip Larkin is considered by some to be the greatest modern English poet. That high stature notwithstanding, there has been increasing attention paid to the personality of the writer, with some accusing him of being a bit of a racist and misogynist. I personally find it very hard to enjoy a writer's work if I know about some of their odious views (although I still enjoyed this book). But if you're interested in reading more about Philip Larkin try this Guardian article here.