The prompts are slightly tailored towards a textbook but are otherwise generic. If you have a Gemini API key, you can specify any title and get your own book. It can take several hours for a larger book (can be done faster and slightly cheaper with async queries, but I thought that an unnecessary complication for a small pilot). It is easy to disagree on the resulting quality of the book; for me it feels more like an extended Google query than anything. But as an exercise where the size of your prompt stack is larger than 1, it was fun and, in parts, thought-provoking.
If I am to name just one prompting trick that I discovered to be useful, that'll be hierarchical budgeting. I have a total word budget for the whole book and I ask Gemini to allocate that budget for chapters, and so on recursively. Fun part, the sum of parts does not always equal the total, but the difference was never bigger than 5%. That's some stochastic summing happening.