Pressbooks is more of a platform than a plugin. In the multisite environment, each site is a book, which allows a publishing house, an educational institution or any organization to create a Pressbooks network that can host a large number of books. Pressbooks would not scale for these kinds of users if a new install had to be created for each new book. I hope this clarifies the technical requirement for you. If you wish to author a single book using Pressbooks you still have to install it in a multisite environment. Alternatively, you could use our hosted Pressbooks.com service. Thanks for your interest!
