37 Signals latest product, an eBook(PDF) called Getting Real. Which, IMHO is more of a way of thinking vs. a concrete formula for development. Much like their web applications Getting Real is simple and articulated well. At $19 it’s a must have for anybody doing development whether or not it’s Software related.
I can attest that the ideas represented in “Getting Real” work. I have been using some(not all) of these ideas in my own job as a software developer for a long time.
You can purchase Getting Real which is a 171 page self-published PDF from the 37 Signals web site at, https://gettingreal.37signals.com/ for just $19 dollars for single license or $49 for a site license.