1. 'Double click to close' is very non-standard web UX, I'd recommend not trying to innovate on UX and simply supporting the back button as sites always have. Double click also breaks text highlighting. It's really only worth breaking the rules once you've mastered them AND have a use case where breaking it opens up a new opportunity.
2. Right now when you press back when looking at a book, you return to the home page rather than the page you were just on (subject area, e.g. Sci Tech), which is also unexpected.
3. If your book info overviews are super short, you may want to consider opening them accordion-style within the subject view, so you can continue to browse/scroll without further navigation.
4. You may want to consider using a standard CSS framework like Bootstrap or Tailwind until you're more comfortable with frontend design. There's an order of magnitude more complexity and nuance to frontend design/dev than engineers typically are aware of and these frameworks get you far, fast.