[He ends up setting the books on the desk, pausing to study that portrait he's hung up. Huh. It looks sort of familiar...]

Hmm? [Oh yeah, he's supposed to be helping. Yuuri pulls out one of the books from the small stack and sets it aside.] This one helped a lot. I didn't know much about space in the beginning either, and this explains the basics.

It's in English, though... I'm guessing you don't know that language?