If your library has purchased a book with a limited-user access model and all copies of the book are being read online by other users or are currently full-downloaded, then:
When one of the users of the book does one of the following, the status of the book will change to: Available
If your library allows the last or only copy of limited-user books to be full-downloaded, the Detail Page will indicate expiration information for the nearest expiration.  For example:
User-added image

There is not currently an option to reserve a book or to be alerted when it becomes available.  If you are waiting for a copy of a book to become available, you'll need to periodically refresh your browser to show the book's current status.

Note that saving a book in your Bookshelf does not guarantee access to it, it simply saves a link to the book.