Just to clarify, I didn't have the same idea, I had the idea that I wanted to make sure I was learning the material, I'm not sure/convinced a quiz is the way to do it, but at least you are doing something. I just had a thought and too many other projects :)

There may be a lot of book recommendation products out there, but name one. I'm not suggesting you build a book recommendation product but as a side-effect of your product, you could help people to buy more books.

The difference between blinklist and bookgym is that they are selling access to the content. Sure, it isn't the real thing, it's like they are selling a knock-off. Not as good as the real thing, but you're getting it cheaper.

For one, I wouldn't spend $10/year for what is currently there. And I don't see how you can scale without crowd-sourcing, so you've got a bit of a chicken/egg problem where people won't want to pay until you have content, but you won't have content until you have people supplying it, whom you're expecting to pay.

I completely agree with you not creating a ton of quizzes at the moment, until you see the market is interested. That's the right thing to do. But you also have to be able to measure market interest, which is a bit difficult to do without content. I don't have a clear answer for you there.