Bookshelves are hand-curated collections of eBooks that have similar topics. Bookshelf listings were created by volunteers, and have not been updated since 2020 so there are many books without these hand-curated bookshelf categories.
Another way to find eBooks related to a topic is to use the “main categories” page or to browse the Bibliographic Record section of any eBook’s page. Most have one or more selectable Subject and LOC Class entries, which take you to other books in the same category.
These are all specific bookshelves existing in Project Gutenberg.