Dan talks about writing books in response to a Chris Anderson post. Since I've been writing for several years it's worth confirming what Dan said.

1. You won't get rich. I've managed to make a living from writing because I do it full time and I've been lucky enough to collaborate on some successful books. But the market has been way down in the last couple of years - the Wrox collapse was partly due to that. Computer books also don't sell in the same numbers as Harry Potter. Even the few definitive books (Essential COM, Code Complete, etc), aren't in the million seller league. At the moment there's debate whether the book market is in a good enough state to allow me to continue writing full time.
2. You also won't get famous. Sure Alex & I have a bit of noteriety, but I suspect that's more to do with the sheer quantity of books. Or possibly the wigs and silly costumes. Speaking at conferences is good, and I enjoy it (in fact I'd like to do more), but fame isn't what it's about. Respect from peers means more to me.
3. Teaching is good, and is probably why I do this. There have been a couple of characters in my past that really made me enjoy the learning experience, and one in a job who always took time to help. I've never forgotten how useful that was to me, and since I'm fairly good at explaining things and enjoy writing, it seems a natural career. I didn't start this way (I spent 10 years as a developer), and in some ways just fell into it. But it suits me. I also don't really see myself as a teacher that much, more of an enabler. I like working with beta products and having a book ready when the product launches, taking the learning pain away for others. I live in beta land, a strange place inhabited by people with no life, and a severe lack of documentation.
4. Dan didn't talk about this much but it's a heck of a lot of work. You may say it's easy for me, sitting at home with the time to write, and that's true. But this is my job, and it doesn't pay well. Besides, I wrote my first two books when I had a full time job. Working 9-6 during the day and then another 5 or 6 hours every night hurts. I have huge respect for writers who also have proper jobs.

Err, enough rambling I think.