During my childhood it was The Secret Garden by Frances Hodgson Burnett and The Lion, the Witch and the Wardrobe. More recently Elizabeth is Missing by Emma Healey.
I found Born on a Blue Day by Daniel Tammet haunting, but possibly because I met the author at a book festival and it was really the first time I had a proper insight into Asperger's Syndrome, which I now realise has featured greatly in my family, mostly undiagnosed.
I ought to also mention A Voyage to Arcturus by David Lindsay, a book which is memorable not for the writing so much as for sheer weirdness. Probably a bit of a marmite novel but it has stuck with me.