In the late 1980s, I read a short story, probably intended for children or young adults and having several sketched illustrations (in blue ink, I think). The whole story took place inside a giant space monsters. The protagonist spent the story traveling around inside the creature, after it ingests his spaceship. There are occasional short paragraph's from the monster's (stupid) perspective as the hero makes his way toward the thing's heart/brain complex. There was a final confrontation there before the dual organs. There was even a picture of the vast fleshy cavern containing them, with the heart many meters high and with arteries wrapped around the similarly sized brain sitting next to the heart.
Any idea what this was?