I have noticed that at times, when I am having too many books, I make up a mess. Also, I want to be able to easily add books which I have bought or downloaded from some other source. Like, I may have books downloaded from Project Gutenberg as well as general books bought from the Kindle store, Kindle Unlimited or some PDFs too.
I like to use Calibre to manage and convert ebooks. You will need Kindle's software for PC in order to download the books, that you can then import on Calibre.
Beware that the books are DRM protected, so you won't be able to convert those.
It is free (both as in '..speech' and in '..beer') and actively maintained.