Loads that have been mentioned
used to love anything to do with magic or fantasy and adventure
not sure if these are listed but
Half Magic Edward Eager where the children fiond a coin which grants wishes but only half wishes so they are half invisible etc
Treasures of the Snow Patricia St John
Fell Farm books
The Cherry family series by Will Scott where they had a wonderful Dad who created "happenings" I adored those books
Wizard of Earthsea, Dark is Rising, the Guardians etc
E. Nesbit
Minnow on the Say etc
Richard Hughes High Wind in Jamaica
Swallows and Amazons
Enid Blyton especially The Naughtiest Girl in the School, and the ..of Adventure series
Fairy Stories of any description..The Ugly Duckling, Rumpelstiltskin. Sleeping Beauty,
The Snow Queen loved it
Emil and the Detectives
Carrie's War
Marianne Dreams
Narnia series
Alan Garner especially The Weirdstone of Brisingamen, The Moon of Gomrath, Elidor
Jill Pony books
And there were a series of travel books with two children visiting New York, Paris etc