Here are 10 books of that kind. Thinking about this brought up a couple which were really important to me but I haven't really blogged about before.
The Tale of Samuel Whiskers by Beatrix Potter
Comet in Moominland by Tove Janssen
Hiawatha by Longfellow
I was pretty young - 8 perhaps - when I read this novel-length poem in very heavy trochaic tetrameter. I had no idea it was not the type of thing to read. I think almost any young kid would like it.
R is for Rocket by Ray Bradbury
I think it must have been 'R is for Rocket'. It was a short story collection which included 'The Golden Apples of the Sun' and 'A Sound of Thunder' - my dad owned it. I know Bradbury has his detractors but I thought this was really well written.
Red Shift by Alan Garner
More than Human by Theodore Sturgeon/ Watership Down by Richard Adams
To make this a list of 10 I have amalgamated these. I love books like this about bands of comrades.
Wuthering Heights by Emily Bronte
The first 'literature' novel I read - to my surprise it was good
Fire in the Lake by Frances Fitzgerald.
This was an early account of the Vietnam war, first published in 1973 (I read it a year or so later). Some people think it is conservative/essentialist ('Asian people think like this...') while others think it is radical/loony lefty ('The Viet Cong were an effective force working from a coherent philosophy, not from evil'). It introduced me to Marxist theory, cadre politics, anti-imperialism, and east-asian philosophy. I don't know how it would stand up to a re-read.
The Larousse Encyclopaedia of Mythology. I think it was the 1959 edition.
A comprehensive overview of a wide range of mythologies from around the world. I just can't begin to judge how great an impact this had both on my imagination, and in making me feel that all religions are contingent. If my parents had had any idea they would have banned me from reading it. I think this was the single book which shaped my childish mind.
The Inheritors by William Golding.
ETA - Oh, I thought of another one, which very strongly influenced my imagination:
Chocky by John Wyndham.
A young boy has a resident alien telepathic intelligence inside his brain. I found that imagining I was describing the events of my life to an imaginary alien really livened up boring schooldays.