"Random Harvest" by James Hilton.
If you have ever seen the film you will know what happens, but the book is a different experience anyway. I love it and reread it from time to time. It's a free one though I can't remember where I found it. Try Gutenberg and Manybooks.
Edit: No, not in those, but in Gutenberg Australia on
http://gutenberg.net.au/plusfifty-a-m.html