Last month I mentioned a free chapter of Steve Krug's new book - Rocket Surgery Made Easy. I've come across another.
Opening remarks: "Call me Ishmael - How this book came to be, some disclaimers,and a bit of housekeeping" (PDF download)
Chapter 4: "What Do You Test, and When Do You Test It?: Why the Hardest Part Is Starting Early Enough" - article on www.peachpit.com