If you want to keep those publications separate from your journal issue, currently, the only way to do something like that would be to use the Static Pages plugin.
If I remember correctly, the pages can be navigable by using header tags (not sure if h3 and h4), otherwise you'll have to add the navigation manually.
This would provide a general page, that would work as a Table of Contents, linking to the abstract pages of each book review and case study, in order to keep page sizes small, and maintenance less cumbersome (updating the TOC after adding a new review/case, for example).
The Static Pages are published in a "higher" level of OJS, which means you can add a link to the main navigation pointing to the Case Studies TOC and another to the Book Reviews (if you want to separate them, for example, allowing for an easier maintenance as well).
I believe the Files Manager within OJS will enable you to upload your files to a "public" directory, but I'm not 100% sure.
I was just testing this but the PubliFolderPlugin isn't working on my 2.3.6 test install (upgraded from a 2.3.1-2).
You may need an FTP/SSFTP tool to upload the files to your "public" folder.
You'll have to play with the plugin to figure out the best way and if it does indeed fit your needs. It will all depend on your information structure and how much content you have now vs. how much content will be published, and how soon does the content grow. A structure that works now may not work in the near future. For example, a single TOC page with all links to Case Studies and Book Reviews may seem the best and quickest approach, but in a few months you may have to separate them because the TOC is very long.
These documents won't be searchable through OJS search, although they will probably be indexed by Google and similar.