Horse & Pony Books:
RIDER BY NIGHT
(1st USA edition HARCOURT, BRACE & CO [USA] 1960)
ILLUSTRATED BY CHARLES W. WALKER
Reprinted in paperback by Scholastic in 1971.
No information for original Swedish publication.
EDITION PICTURED: Paperback edition
SUMMARY: Set in Sweden. Jenny has a lovely mare called Rascal whom she keeps at the local stables. When Rascal starts displaying some odd habits, such as a tendency to be tired all the time and an attraction to the colour red, Jenny is determined to get to the bottom of the mystery.
The paperback edition is easier to find than the hardback, though that edition is not particularly rare either. Easy to find in the USA and fairly easy to find in the UK. I presume also easy to find in it's native Sweden. A bit harder elsewhere.