I would argue that, being stylistically inconsistent with the way most apl is implemented, that's actually not such a great resource. I found 'whitney c' surprisingly approachable, with just a little patience; despite no prior experience with it, I was able to make an apl-ish interpreter in 7kb and a few hours. If you're curious about implementation details, that might be a more worthwhile exercise. Be sure to check out the parsing section[1] of the J dictionary.

1: https://www.jsoftware.com/help/dictionary/dicte.htm