Sheesh... doesn't anyone look through the existing threads before posting?
See this thread for more information on your issue:
500 error when adding HTML page from archive.org.
Short answer is you are trying to download books without being logged in. Notice the 'borrow' in the URL you are trying to use.
Even shorter answer since World War Z is still in copyright, you are going to get very little help with pirating ebooks in these forums.