Normally this error implies a session or cookie issue.
Please try signing out of your bookshelf/account then clearing your browser cache and then sign back in and retry opening the Online Reader.
Information on how to clear your cache can be found here.
It can also be caused when people try to access an ebook directly by saving a link to the Online Reader (for instance saving it as a favorite). This will not work because you need to be signed in to eBooks.com in order to be authorised to access the ebook.
To access the ebook correctly, go to eBooks.com in your web browser and sign in to your account. Then in the My Bookshelf section of your account you will see the ebook and can click "Read now" to open the ebook in the Online Reader.
If the above solutions do not work and you receive the AER5990 error again please contact our support team. Please quote the error AER5990, tell us which of your ebooks is experiencing the issue, and explain you already tried the solutions above.