I've gotten two of these mails and still do not understand what they're asking me.

So I see three options:
1. they're just bad at explaining; or
2. their lawyers told them to keep it vague; or
3. they're doing malicious compliance and/or lobbying by making it intentionally confusing, so authors and publishers blame the EU

(3) would then help them with other aspects of DSA that are unrelated to books.

If I had to bet it's (1) though - the iBooks department is probably too low on their list of things to spend any attention on, so they don't bother making this compliance step easy to understand. 

 I mean: a what does "trader" mean here? I wrote a book. I paid for my editor and other things as a business expense, so I consider royalties to be revenue. Does that make me a "trader"? What even is that? Why does the EU want to know that? 

 Doesn't it point you to a link that's supposed to help you decide if you're a trader or not? 

 Oh yes! https://itunespartner.apple.com/books/support/5401-manage-digital-services-act-compliance-information#trader

(which about as helpful as the email, even linking to the EU page about the entire DSA) 

 Oh yes! https://itunespartner.apple.com/books/support/5401-manage-digital-services-act-compliance-information#trader

(which about as helpful as the email, even linking to the EU page about the entire DSA) nostr.fmt.wiz.biz 

 > Trader self assessment: Find information about how to know if you are a trad

Leads to:

> Apple can’t determine whether you’re a trader.

And:

> To determine if you’re a trader, you should consider a range of non-exhaustive and non-exclusive factors (see those listed on page 2 in the EC’s Guidance), which may include .... 

 Would you say this is bad on the part of the EU or Apple though? I don't really know anything about this. 

 I don't know, that's what I was asking about.

In any case if the answer were "yes" (trader) then Apple has to doxx me. So I'd remove the book in that case.