April Dunford's book, Obviously Awesome, is very good and worth reading if you're a founder or marketer (or both.) We reverse engineered her workshop and did it as a team at the end of last year for my startup. Was very much worth the day.
Accessible to founders, and helps lay out the art and science of positioning, and give a methodology for actually doing it. Would recommend it also to technical people who are sceptical of marketing in general.
Byron Sharp's How Brands Grow is also a great book from one of the world's leading marketing professors, and aims to lay out some iron laws about buying behaviour. Byron is quite a spiky and contrarian personality, which makes the book a lot of fun to read, but he also really knows his stuff.