Which engine should I use for long audiobook chapters?

Use the Audiobook Studio rather than the plain TTS studio for chapter work: it holds one voice across every chapter and lets you re-render a single line without touching the rest, which is what protects consistency over hours of audio. Engine-wise: Gemini Flash where you want acted delivery, Kokoro for fast drafting passes that cost you nothing. Export gives you stitched per-chapter WAV files ready for the audiobook platforms.