Five years ago, a couple of colleagues and I began writing a Sage-based textbook to serve a class we teach at our institution. When we announced it to Sage users, we received an encouraging reception and excellent feedback. If that was meant to discourage us, it failed completely. ;-)
We've updated it pretty regularly since then, correcting a lot of errors and adding a few new features, even updating to Python3. The sources has been available online for a while, but after half a decade it seems time to get a little less behind the times than we have been and move the entire project to GitHub. So, here you go:
A new PDF version is included as a "Release", so you don't have to clone it, let alone build it. (Look for "Releases" on the right.) The license is CC-BY-SA, so feel free to clone it, fork it, commit it, push it, and any other unethical-sounding VCS operation that suits your fancy. You can even introduce errors that we haven't already included!
To honor the occasion we changed the title. Two of the authors are very pleased with the acronym.
We hope people find this useful for teaching, learning, and using Sage. People besides us, that is. :-)