...when you've finally hit on The Book:
You contemplate leaving early from a night out with your co-workers because you're getting that itch to write, and you can't stop thinking about the work you could be doing at home. Heh.
The book is going well. I am very, very pleased with my progress so far. I'm not going to say much about it, since I suspect the second draft will differ a lot from the first, except that I've hit my stride on it and we're starting to run.
I'd forgotten that I decided I wanted to write because of times that felt like this. :)