Writing a company-wide developer's handbook

Question

I work for a small company. The software development arm of the company before I was hired consisted of one self-taught overworked guy. Now that I've been writing software for the company for a few years, I have been tasked with establishing formal company-wide software development practices. We currently have no guidelines, other than

Write code, test it, put it in a .zip file and send it to the client. Bonus points for TDD and version control.

My boss wants me to write a software developer's handbook which defines the general processes, protocols, tools, and guidelines we use to get things done. In other words, he wants a "This is what we do here" book to make it easier to get a new employee familiar with the way we do things, as well as to help my boss understand what his minions are doing and how they do it.

The way I see it, I'm laying a foundation and it needs to be done right. How would you go about choosing topics for such a handbook? Can you provide some example topics?

Side Note: If it matters, we are primarily a Microsoft .NET shop. And we are looking at agile practices such as XP and Scrum, but we may have to heavily modify them to make them work in our company.