My free OpenRouter model stopped working halfway through the book and BookyAI continued with Gemini. Can I choose which model takes over instead?

Yes, from version 1.9.13. In Settings, under "OpenRouter fallback model", pick a second OpenRouter model (a paid one is the safest choice). When your main OpenRouter model fails during a book (rate limit, empty answer or an outage), BookyAI tries the fallback model first; only if that fails too does it move on to the Gemini rotation and Ollama, as before. Each chapter shows which model actually wrote it, so you can see when the fallback was used. Leave the field on "None" to keep the previous behaviour.