While I believe youtusba doesn’t have a purchasable ebook in japanese I do believe it is readable online. In a legal way I mean. Kadokawa group hosts several of the chapters of it online in japanese, or at least they have their company name all over on the website. Kadokawa is the publisher for Evangelion, so presumable they own the rights or have an agreement with the creators of yotsuba. They don’t host all of the chapters so it might not be worth even mentioning but
I’ll have to review the website owners demands for link sharing before posting a link to make sure it complies. If I am pretty sure it does I’ll provide a link. Either way I’m very confident that at least some of the book can be viewed digitally and legally.
Edited: I do believe the links comply with the " or any other legal source" part of the rules.
There is the website, it has the sample chapters which you can view. It’s primarily a store but it does link to a few of the starting chapters.
@Asher Please verify I’m not breaking to many rules here