This thread reminds me of how my sister recently told me that she almost cried when she'd finished the Children of New Forest as a child because she would never experience anything as wonderful again.
My mind was blown by
The Brothers Lionheart as a child. It was what finally made me want to be a writer (even though I sensibly waited with trying to write a magnus opus until I'd grown a bit). It's by Swedish writer Astrid Lindgren (of Pippi Longstocking fame) and is probably not very well-known in the English speaking world but well worth looking into (and rumour says it's currently being filmed by Tomas Alfredson, who made
Let The Right One In and
Tailor, Tinker, Soldier, Spy so it could be a really awesome version).
Later on,
Pride & Prejudice blew my mind - it was the first book I read in English and I was about 15. Jane Austen's style just exceeded anything I'd read up until then, and it was definitely a huge marker in my reading career.
Later on, Dunnett's
The Game of Kings also blew my mind, because it was so wonderfully rich and I loved how cleverly she played the reader (I didn't like the following books in the Lymond series nearly as much). But I think the book that really blew me away as an adult was A.S. Byatt's
Possession. I loved the blend of story-telling and fictional non-fiction and it was just so wonderfully rich too - it almost put me off writing, because I could never manage anything like it.
In the last year, William Dalrymple's non-fiction work
The Last Mughal also blew my mind, simply because he writes such exquisite prose. Reading him is a lesson in style that's useful for novelists as well as non-fiction writers and I'm eagerly awaiting the arrival his latest about the First Afghan War (which I bought the other day).