The illustrations in Westerfeld's work has a designed effect on the reader. As he was composing Leviathan, it became clear that the illustrations were needed in order to create a designed effect upon the reader:
I’d written the first 16,000 words as a regular novel, but then I was trawling for visual inspiration from the rich vein of DIY steampunk culture, and realized that the book itself could look like an object from the period. Most novels were illustrated back then, and quite handsomely, and Keith has reverse engineered that style. The result is a sort of “Victorian Manga” look, based on Punch magazine illustrations in 1914. It feels very old-school, but it’s still accessible to the modern eye.
Given the genre of the work as representative of Steampunk culture, the illustrations were essential. Steampunk elements feature mechanized, alternate histories of the British Victorian era. Such a context needs to be illustrated to be appreciated. Stark images of gray, dark, and mechanized reality can only be fully understood in illustrations. This also matches well with the context of the novel as an alternate history of the First World War. Westerfeld employs illustrations to ensure that the full force of both the genre and the context is evident.
At the same time, the illustrations used help to place the reader in the midst of the novel's world altering context. Westerfeld alludes to the "Victorian Manga" look of the novel, a deliberate intention on the part of the author. He wants the reader to feel as if they are in the midst of a larger than life epic struggle, an apt description of both World War I and the alternate history that is the book's settings. The illustrations identify the exact forces that Aleksander and Deryn face. Illustrations in the style that is employed help to convey this reality to the reader and create a specific effect in the reader's mind.
Finally, I think that the illustrations are designed to bring into the mind's eye what the characters experience. For example, when Deryn ascends, one sees the exact force this moment has on her. Westerfeld writes that there is a certain excitement that she experiences when she is about to ascend: "She couldn't wait to get off the ground, the flightless years since Da's accident suddenly heavy in her chest." The illustration of "Ascending" helps to illuminate this condition. It shows Deryn completely above the world that she was once a part. She looks with down at the ground with a sense of exhilaration at being airborne, but also with a sense of anxiety, knowing that her life will never be the same and that she is actively going back into the past, a real, where her father's life was claimed. The tentacles swarming above her provide almost a disquieting foreshadowing effect on the reader regarding the complexity that she will face and that the world will face as a result of the war. Another example is the illustration of the Leviathan itself. Westerfeld's descriptions are visual even in text:
The Leviathan had been the first of the great hydrogen breathers fabricated to rival the kaiser's zeppelins. A few beasties had grown larger since, but no other had yet made the trip to India and back, breaking German airship records all the way. The Leviathan's body was made from the life threads of a whale, but a hundred other species were tangled into its design, countless creatures fitting together like the gears of a stopwatch.
In the illustration entitled "The Leviathan Approaches" the visual image that Westerfeld offers becomes real for the reader. This effect is absolute. In the text, the idea of something that could rival a zeppelin helps to conjure immense images in the reader's mind. The description of the vessel as consisting of a whale and "a hundred other species" that were tangled is enhanced through the illustration. Seeing the head of the breather in the picture as well as its markings, indicating where other species' life threads exist, and finally, the small eye that helps to distinguish it as a whale casts a distinct effect on the reader. When the reader recognizes the Westerfeld image in illustrative form, there is a clearer understanding that emerges. Westerfeld suggests this condition is deliberate and is designed to cast an impact on the reader: "I’m a pretty visual writer, but this process [illustrations in the text] brought home how much we novelists are cheating in our heads." It is this effect on the reader that becomes one of the most important elements regarding the illustrations in Leviathan. The illustrations allow Westerfeld's images to become actualized outside of his own mind and inside the reader's.