x2rm5,
I am not the technical guy here, but do me a favor and give this a try:
The site (for me and many others) has some idiosyncrasies. One of them is that, when clicking to go from the listing of books to a specific books download page, the session information of who you are does not properly load, hence the error message. I found that if you navigate back one page, hit the F5 refresh button, and try again, the problem goes away.
If you would, please give this a try and let me know if that works for you.