Did life start from simple chemical reactions? What is the nature of Life? I was reading about these questions in an excellent summary of the current ideas at Scientific Curiosity blog.
I bought a book many years ago and still haven't managed to finish it: Investigations by Kauffman. Nevertheless, I did read the chapter on autonomous agents (translation: Life) and his definition has stuck with me all these years: "a self-reproducing system able to perform at least one thermodynamic work cycle".