Lots of good suggestions here. One I don't see is Wotdke's "Radical Focus". It's about OKRs, but don't be put off by prior big-corp experiences with it. The book focuses on using them in an entrepreneurial context and it's great. Even if you decide to come up with your own system, this will help clarify what you want out of a way to set goals and make sure you're on track for them.