Demons Don't Dream, by Piers Anthony
Teenager sucked into a video game (using the "calibrate your eyes" technique) where trying to subvert censorship has penalising consequences, and he picks an irrationally buxom NPC, Nada Naga, to accompany him. He fails an initial challenge by peeping on her (trying to be clever by viewing her only by reflection off the water).
The novel is part of the Xanth series, and was released alongside an actual computer game set in that world.