It’s just called 國文一! It’s published by 南一書局, which is how you can find it considering that textbooks published for schools usually have such generic names. I purchased this in 2009, though, so while the publisher is still around and publishes 國文 textbooks, I’m not sure how much they’ve changed since then!

I’ve just paused the challenges over the summer. We’re working on a new version of the site and I thought that taking a break and launching the new version after the summer would be the best way to do it. Regarding textbooks, I think what you say about non-everyday characters is partly (or even mostly) what makes these books difficult for second language learners, especially those who haven’t really mastered the everyday characters yet.