iTunes on Windows has a problem where it will lock up after a number of files have been copied to the iPad. It can sometimes run out of memory or crash and can be general buggy. DiskAid has been reported to fix all of these issues when copying files to the iPad. It is a relatively low cost solution to the iTunes lockup/freeze problem.
Connect your iPad to the computer.
In the left sidebar of DiskAid, select Storage->Apps->unrealBook
Drag files into the files listing to the right of the unrealBook icon.