Book recommend. Rapid Development by Steve McConnell. Actually I can heartily recommend everything that I have ever read that he wrote, but that book addresses the topic at hand. Including why a spec is nice, what a good spec should be, why you are unlikely to get one, what can (sometimes) be done about that, etc. A lot of this may sound like management theory. But it is really addressed as management theory for a technical lead (a role you can find yourself being thrown in unexpectedly - or be in by default by virtue of being the only developer) who would rather be a programmer but needs to know what works out of self-defence.

Another approach to the issue of the fact you will never get a spec is to try some of the ideas from extreme programming out. If you search you will also find some previous discussions of that.