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:

• In search results, the book will be shown as:  In Use
• And on the Detail Page, the "Read Online" button will be grayed out

When one of the users of the book does one of the following, the status of the book will change to: Available

• Signs out of their Ebook Central account
• Returns to searching or opens a different book (in the same tab)
• Closes the tab or the browser they were reading in
• Is idle with the book for 30 minutes (that is, no page turns, copies, prints, etc. for 30 minutes)
• Reaches the end of their download period for full document downloads

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:
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.