Tim's wealthy parents have been spending lots of money on his education to make him into a successful adult--good boarding school, where he is prepared for Oxford like a goose being fattened, to be followed by joining his father in the advertising business. But Tim derails things by getting sick, with what sound like mono, and having to take a break from school in the new house in the country his mother thought she wanted. The remains of an old house were mostly demolished to make way for the new one, and Tim claims the one little surviving bit as his own room, which his mother doesn't understand (the first of many such no understandings in the story...). For the first time in years, there is no pressure on Tim, and so when a builder working on the old chimney in Tim's room finds a box full of old drawings hidden away, Tim has the chance to reflect on them at leisure.
Impossibly, inexplicably, the artist, a boy called Tom, starts to become real to Tim. He knows things about him he couldn't know. And he wants to know more about Tom, and the girl, Netty, he drew. He finds Tom's gravestone in the churchyard, showing that Tom died when he was just about Tim's own age back in 1910, and there he meets the vicar's daughter, Rebecca (also dealing with heavy parental expectations), who becomes his companion in both looking for Tom and Netty, and in figuring out what he wants to do with his life.
The story in the present is interposed with Tom's story in the past (trading school when still a kid for the hard life of a farm laborer, though still finding time to draw). It's not a time travel book, because there's no travelling, but there is time slipping in the connection between the two boys, which is lovely and magical, and a nice counter note to the sadness of Tom's story in the past (wealthy, self-centerdly oblivious Netty is a piece of work, and there is tragedy) and Tim's struggles in the present. Peyton's descriptions are utterly beautifully vivid, adding to the magic of the story. And it's great to see how Tim comes into his own.
Though it is set around 1970, the narrative of teenaged emotional growth is as germane today as it was then. It would have been a young adult book back then, with its bit of romance and the rebellion against parents (and it's very 1970 YA cover art), but I think the most appreciative audience, then and now, would have been dreamy, imaginative 12-14 year olds.