These ideas sound rad! And having a cargo bike is just fun anyways. You might look up the Halstead's rack weight limit, IIRC it's something like 50 lbs? But I could be way off.
If it were me, I think I'd just throw a cooler on the front, load it up with sodas / juice / water from costco, and pedal to the most crowded public spot I could find to sell them. Just to see if it works, kind of thing. Then maybe start into more niche stuff.
Bicycle delivery was popular in the city I lived in last, but it almost seemed like people bought it because they liked the idea, not because it necessarily made sense. And I'm not trying to be salty about bikes! I was a bike mechanic for years, and I love utility cycling. But a lot of times, the speed issue seems like it'd be hard to get around -- people want their growlers now!!! I suppose if you're in a dense part of the city, my pessimism might be unwarranted. I'd just keep your mind open to opportunities where having a bike allows you access to areas that cars can't reach, like the interior of a park or something.
Anyways whether it's books or something else keep us posted! I really hope it works out for you.