A science-fiction novel, or rather a logical fantasy, by John Wyndham. First published 1968, currently published by Penguin, ISBN 0-140-03121-9.

"Polly, their daughter, had Piff, who, with her empty seat at the dining table and late-night demands for water, was a typical invisible childhood friend. But Matthew, their son, has Chocky. And Chocky seems to be well versed in higher mathematics, used to a thirty-two hour day and can offer a suprisingly coherent account of how a gravity-shielded, cosmic-radiation powered spacecraft might be made to work..."

The blurb on the back explains the plot so much better than I could. I wasn't too impressed with this, one of Wyndham's later novels, but nonetheless it provides some thought-provoking concepts, as always. Once again the idea of fear of the unknown is brought to the fore and dealt with in a somewhat more mature manner than in some of Wyndham's earlier works. And the ending is very sweet.