Professional indexes often include metadata.opf files or sidecar files showing ISBN, publication date, and author details. This allows tools like Calibre to automatically organize your library.
If you are looking for digital collections known for maintaining high standards of formatting and utility, several platforms are recognized for their quality: index of ebooks high quality
If you are looking for specific academic or diverse topics, these "meta-indexes" point to multiple high-quality libraries. Professional indexes often include metadata
(as you wrote):