I have no idea what happened to most of my childhood books - presumably they got given to charity at various stages. Including, I suspect, the lovely old illustrated dictionary that I used to spend hours poring over as a child. Initially it was simply because of the pretty pictures and the amazement that a book could have dents in the pages to put your fingers in, then later on for all the other bits that went with it - the words, the look and feel of it etc. I know some of the flora and fauna and bird guides I had are still with my parents - my mum won't let me have them back because she likes them too much...
Most of the fiction and other non-fiction, though, I suspect has long gone (I read anything I could get my hands on, so as well as things like the Famous Five I was reading James Herroitt, Joyce Grenfells, a range of biographies and autobiographies, and whatever fiction my aunt and nanna had bought or borrowed from the library and left within reach).
Actually, from my teens, I do still have my copy of Tschiffely's Ride. And a very badly bound and edited hardback edition of all of Jane Austen's works.