How do you design? A Compendium of Models by Hugh Dubberly

An excellent 150 page free download.

Subject To Change: Creating Great Products & Services for an Uncertain World: Adaptive Path on Design by Peter Merholz, Todd Wilkens, Brandon Schauer, and David Verba

The Stages of the Usage Lifecycle. From Designing for the Social Web: The Usage Lifecycle

-and-

Designing for Sign Up (video & slides)

Web Form Design by Luke Wroblewski