A short sketchbook that came with the limited edition of Volume 06. This is the second artbook printed. The first can be found on a previous upload, and the following artbook is a really expensive limited edition physical print that was part of a $150 box set.
News about the limited physical print: https://natalie.mu/comic/news/332626
There was also some card game: https://www.animenewsnetwork.com/interest/2019-05-24/witch-hat-atelier-manga-5th-volume-bundles-original-card-game/.147036