If you are unsure whether a book is in the public domain, a quick search can clarify. In the US, works published before 1928 are generally in the public domain. If a book is still under copyright, downloading it from a source other than a licensed vendor or legal digital library is likely a violation.

The fact that a file is on an open directory does not mean it is free to download or distribute legally. Alternatives to "Index of" Searches

: A specialized index for technical manuals, programming guides, and math textbooks. The University of British Columbia how to create a hyperlinked index for your own eBook or document? Cite ebooks - Cambridge LibGuides

The most effective way to find these raw index pages is by using specific advanced search operators on search engines like Google.