That's impressive work! But how come the books are in mixed languages? Are they your books? Do you fix books for random people?

Hahaha, good observation :) I ran out of my childhood books (in Spanish) to fix, then I ran out of my GFs childhood books (in Latvian), so at that point I offered my services to anyone in the Google Zürich office. I got books in all kinds of languages I couldn't read, and a few chocolates and bottles of wine for my troubles :)

Some of the most interesting books were especially challenging. One in old German that was missing a couple of pages. It was a popular fairytale so I found the missing content online and a closely matching font, and reconstructed the pages. Another was in an Asian script I not only couldn't read, I didn't know how to sort or even properly rotate some loose pages, so I had to ask the owner. In a few cases when bits of the cover were missing, I found photos online, and printed a patch. Fun times!

that seems wonderfully varied and entertaining :)

Hi! I've been interested in starting book restoration as a hobby as well, are there any resources you recommend to start learning?

I learned primarily by trial and error, haven't found any good articles or videos :(