2x2网格书籍封面设计：cel-shaded插画风格

2x2 grid, 16:9, do this for 4 non-copyrighted books: {$SUBJECT, flat cel-shaded illustration with heavy uniform dark ink contours, engraved technical-plate linework, panel seams and fine interior hatching, stepped flat fills with hard shadow terminators, desaturated bone and steel-grey base palette, sky graded from cool slate navy to warm amber, electric white and oxide red accents only, broad backlit ambient light, rim-lit silhouettes, no cast shadows, perfectly clean digital surface with zero grain or texture, detail concentrated on the primary mass and sparse elsewhere, near-orthographic perspective, low horizon, generous negative sky, monumental stillness}