Shape Up is how Basecamp does product development. You can see it as a fully-fledged project management framework, and as a toolbox full of techniques to help teams who struggle to ship from which you can pick and choose.
The book "Shape Up" was written by Ryan Singer, Basecamp's Head of Product Strategy, and released in the Summer of 2019. Since then, many teams across the globe have started adopting this way of working. Some as a whole. Others by tweaking it to their context.
If you're interested to learn more, read the book, join the Shape Up forum or attend a Shape Up Practitioners meetup.
Shape Up is available to read online, in PDF and print. Head over to Basecamp's official website to start reading.
There's a very active community in the official Shape Up forum. Here, early adopters share their tips and tricks.