>What's a good place to start for languages like this?

I recommend:

https://xpqz.github.io/learnapl/intro.html#but-it-s-unreadab... (for apl)

and

https://xpqz.github.io/kbook/Introduction.html (for k)

... the author admits they are not an expert in either, but the pace and tone are very beginner friendly, which I have found is necessary if you do not have any experience in array languages.