"37signals’ manifesto and eBook, Getting Real, is available for free as an HTML site. The book details 37signals’ rapid prototyping and web-app
development process with the goal of showing you “the smarter, faster,
easier way to build a successful application.” If you’re familiar with the company’s  Backpack and Basecamp, the ebook essentially answers the question, How’d they do that? 

Designers and studios working with new up-and-coming technology
firms will also find the book useful for getting a better understanding
of the philosophy and working methods being adopted by many “Web 2.0”
companies. PDFs of the eBook are still $19, but the HTML version is absolutely free. Yay!" via lifeclever