Due to the way the new docs site is structured, no books except release notes have an index.html. Instead, https://docs.fedoraproject.org/f26/release-notes/index.html serves seems to serve as the index for each book. That means that if you want to link to the install guide but not to a specific part, you have to link to https://docs.fedoraproject.org/f26/install-guide/Book_Info.html instead. This isn't a huge problem but it could cause some confusion.

Ideally, every published book on the new site should have its own index.html, and upon accessing that you should see the page's Book_Info (or whatever else is the first page) and that particular book's ToC should be expanded (which seems to already happen if you link to Book_Info).