MostlyAdequate/mostly-adequate-guide ? reverse-engineered prompt

Build me a simple online book site for learning functional programming in JavaScript, with a clean reading layout, a cover page, a table of contents in the sidebar, and separate chapter pages that readers can move through easily. I want it to feel like a real tutorial book, with code examples, practice exercises, and a small support section for the helper code used in the chapters.

Please also make it easy to generate downloadable PDF and EPUB versions from the same content, and include whatever setup is needed so the book can be built locally with the right Node version. If you need to check current docs for the book tooling or ebook export, go ahead and look them up.

Keep the style simple and readable, since the main goal is teaching.