If you have lots of books, I wonder if using ePubCheck from the command line would help validate them all.
Take a look at
http://code.google.com/p/epubcheck/ especially
http://code.google.com/p/epubcheck/wiki/Running
Once you have epubcheck installed you could process all ePub's in a directory from the command line as
Code:
for %1 in (*.epub) do java -jar epubcheck-0.9.2.jar %1
assuming of course that directory paths etc were right