Marc Sniukas has put some time and effort into assembling and summarizing the ideas of several thought leaders in the strategy business. He's put this all in a slideshare that's a pretty good read: "What is Strategy?" -- see the embedded reader below (however, if you want to download it, bring money... sharing only goes so far.)
My suggestion is pay attention to the attribution in the small print on each page. You'll find that there are a hand full of references -- some free, some reasonable, some pretty expensive -- that are the intellectual underpinning of this presentation. It's not clear if Sniukas has added any original thought, but nonetheless, this is a good presentation.
And, before I leave you to the slideshare, let me add my own value: here's my personal definition of strategy, as given in my new book: "Maximizing project value"
Check out these books I've written in the library at Square Peg Consulting