Having lots of e-books on your computer (or any other type of file) will not slow it down. A computer mainly slows down when lots of programs are running.
You say you have Norton AND McAfee. Both do anti-virus and both do firewall.
IF you are using antivirus AND firewall on both this is a BAD THING.
Is it only the download speed that is bad or the overall computer speed ?
Remember the download speed is not always that quoted.
For example if the machine you are getting it from is slow, or has a slow upload speed, it doesn ot matter what your download speed is if it cannot get the data quick enough.
Only have ONE anti-virus program running and ONE firewall running.