Forms that Work by Caroline Jarrett is a book that nicely compliments Ginny Redish's Letting Go of the Words. Both sit on my bookshelf.
Web forms are incredibly frustrating when they're badly designed, and can be a barrier to successful eCommerce or even just basic interaction between the web publisher and their readers.
Caroline's website has useful free resources, so there's lots to learn without even buying the book.
Forms that Work - free online resources and book promotion