I keep being tempted by this e-ink device 

I keep being tempted by this e-ink device 

Tempting price, but no built-in illuminator and maybe a proprietary ebook format? Not sure.
And it's opened up somewhat officially for alternate firmware like CrossPoint: https://github.com/crosspoint-reader/crosspoint-reader
I actually was able to increase the performance of a piece of code I was writing tenfold in JS by using recursive arrays…
I was trying to use a thesaurus library to create a list of synonyms and their synonyms and so on and so on, where proximity in the list roughly corresponded to word similarity. I made an object where each key pointed to a sub-object, which had a "synonyms" property that was an array. Within that array were references back to items in the parent dictionary. It was pretty cool. (I'm not remembering the exact structure, but that should be the gist of it.)