To read about how Scratch processes your code, the order it runs scripts and the way Scratch's "threading" system works -
Knowing more about what's going on under the hood will allow you to create better Scratch code and will also shed light on why it seems like some things don't work as they should.
For an absolutely amazing toolkit of useful Scratch additions that I would hate to be without -
This open source selection of tools adds a huge amount of amazing functionality for when you're working on your Scratch projects.
Want to see your Scratch project in a more conventional text-only style?
Scratch and source-control may not seem a possible combination but it's a system I rely on why creating some of my larger projects. You can do this with your own projects using -
Want to auto-magically create your own Scratch profile page?
You can see how this looks on my own profile here.
This useful add-on allows you to add Scratch blocks directly into your Google Docs and Slides -
This is a data-driven website that looks very much like this. Adapt it for your own projects simply by modifying the data.js file. It's an easy way to set up a demo site for a school or coding club.