Illustration style is one of the most consequential creative decisions in a picture book, and it is worth slowing down to get right. The same story can feel warm and nostalgic in soft watercolor, playful and energetic in bold cartoon style, or sleek and modern in flat vector art.
Age range is a useful starting point. Board books and early picture books for very young readers often benefit from simple, high-contrast shapes and limited color palettes that are easy for developing eyes to process. Chapter books and books for slightly older readers can support more detailed, textured illustration.
Tone matters just as much as age. A gentle bedtime story calls for a different visual language than an adventure story or an educational title meant to hold a classroom's attention. Hand-drawn pencil work can lend warmth and intimacy, while 3D character style often reads as more contemporary and dynamic.
It's also worth thinking about consistency across a series. If you plan to write more books featuring the same characters, the style you choose for book one will likely define the visual identity of the whole series — so it's worth reviewing a range of sample styles before committing.
A good illustration process starts with a style selection conversation, moves through character sketches for approval, and only then proceeds to full-spread illustration. That sequencing protects your time and your budget by catching style mismatches early, while sketches are still easy to adjust.