Symbolism, Imagery, Allegory
Shin is never without his sketchbook (1.43). At first he uses it for thorough notes about newfound gastropods and their habitats; and then later he uses it to quantify God (4.84), write the sacred Chutengodian scripture, and draw pages upon pages of towers.
Jason and Shin's friendship originally solidified the summer they collaborated on a comic book. "It was about a bunch of guys fighting aliens on a planet where all the buildings were intelligent" (1.45). So these two came together around making up a world. And a religion is kind of like a world insofar as religion is a way of interpreting the world. This—combined with the intelligent buildings—makes an interesting forerunner to the personification and deification of the water tower.
The drawings reflect things about the drawers (as in artists, not things you put your clothes in) too. As a big-picture guy, Jason's drawings "were always full of drama and action," whereas methodical and scientific "Shin was into the details" (1.45). Kinda like how Jason starts the CTG as a gag without considering any details, while Shin focuses in to the point of obsession.
And through the guys' drawings, we see how they see others. Jason draws a scantily clad Magda (24.1), and when Shin draws Jason's personified tower "the drawing is larger than any of the others, filling the page from edge to edge" (30.67). So we know Jason has the hots for Magda, and we also understand that for Shin the tower is more important to him than most other things.
To an extent Jason's and Shin's drawings are secret, particularly from their parents. It doesn't go so well when Jason's dad finds his drawing of Bustella, and he hides the sketch of Magda; and Shin's CTG writing and drawing grows alarmingly obsessive. In some sense the sketchbook's contents are subversive—especially when Jason reworks the midnight mass with a more desired outcome (24.1)—but that's often the case of texts associated with religions.