When someone asked me a question about udev the other day, I realized I didn't know much about it. I had recently finished the Linux kernel driver book so I thought I should have remembered something. Looking back at the book, I found that the book predates udev and so I really hadn't read anything about it (other than references at LWN and such).
So I went to the udev web site and read the OSL 2003 paper (pdf), which explains udev nicely. I strongly recommend reading it!