Unfortunately you can not download books to an iPad like you can to your computer. The good news is that it looks like some of their public domain (free) books originate from Project Gutenberg and you can download everything from Project Gutenberg through the Stanza app (which is also free).
Hey Peter -- just install Stanza, eReader, or ReaddleDocs and you'll be able to download directly from manybooks.net to your iPad.
Stanza is free and works really well with a whole bunch of free ebook sites. You'll want to download ePub documents with this app.
eReader is also free, and has a lot of nice features (including a direct link to manybooks.net's library!). The format to use with this app is also called "eReader."
ReaddleDocs works with many formats, but ePub would probably be the way to go -- the app costs about $10 but it's very versatile, beyond the ebooks.