They gave you plenty of criteria; they gave you a book, and the understanding that they wanted you to be able to talk intelligently about the material that the book covers. If you're so incapable of self-introspection that you can't accurately judge how well you understand material without external validation, then you should probably work on developing some self-awareness before getting upset that a company doesn't respond to arbitrary requests for meaningless information.
I rephrased you because, if I just quote you, there's no indication that I understand what you mean by the words. You seem to indicate that I in fact didn't. I'm going to call that a success for rephrasing. If I get you wrong, I hope that you'll tell me what you actually meant, which you didn't do here.
Let me try saying things another way. If I ask, "what does 'ready' mean?" and you reply "it means 'ready to move forward in the process'" (my emphasis), you've defined "ready" by reference to itself, which is obviously incapable of being helpful. So I did my best to determine what you might have meant. The entirety of my complaint is that, lacking domain knowledge, and faced with a job ad saying "no knowledge is necessary to apply", I was told "you're not ready to apply, come back when that's no longer the case" and then, upon asking, was not given any guidance as to how to determine whether I was or wasn't ready. So, in answering the question "am I ready to move forward in the process?", all I can do is devolve it into the question "do I want this process to go forward regardless of the outcome (which I can't predict), or would I rather remain paused?". That's purely a question of, in the words I used before, how quickly I want to conclude the process. I can't refer to any other variables or goals, because I can't evaluate those.
> If you're so incapable of self-introspection that you can't accurately judge how well you understand material without external validation [...]
Nobody can do that. Try evaluating yourself without external validation sometime. If you score e.g. a "good", ask yourself "good in terms of what?" External validation is the only thing giving any meaning at all to evaluations.
This is not a hard concept here; I would rate it as slightly more complicated than walking and talking at the same time, and slightly less complicated than walking and texting at the same time. So, uh, you probably shouldn't walk and text because you'll hit a tree or fall into a pit. Just some free advice there.
If you're capable of forming memories, you're capable of self-evaluation without external validation, you goofball. "Good in terms of what"? Most people choose "how good [I was] before [I] started studying" and it seems to work pretty okay for them.