In addition to the resources listed in the manual, you can also check this:
From skimming the resources listed in msteffen’s guide, I realized that Apple used to also offer their AppleScript Language Guide in PDF form—something I’ve really been wanting! But for some reason that’s not included in the documentation archive. The version available there is dated 2016-01-25…
I dug around a little and found various versions of the PDF. The most recent one I was able to track down is dated 2015-09-09, available here in the Wayback Machine. It’s nicely formatted, including an extensive document outline and a ton of internal links.
I imagine the difference between this PDF and the 2016-01-25 web version is pretty small. I like it so much better.