I also did finally get the Cassette Girl book and it's also great. Much long interviews but not english. Lots of lovely Koyama Shigeto designs and artwork. It also definitively confirms the identity of the person in the tube at the end
The scenario and storyboards refer to the person as wearing a red and silver windbreaker, so yeah, definitely Ultraman Anno.
Lastly, as these 4 books are out, the only book left out of the initially promised ones is Kazuya Tsurumaki x Sushio “I can Friday by day!", which should also be fantastic.