I just wanted to pass on a tip – when I downloaded a couple of books and then tried to drag these to my iPad in the left source pane, iTunes crashed, every time. Here’s what worked:
1) download books to the Desktop
2) Drag .epub files into the “Library” in iTunes
3) select the “Books” entry under the “Library” group in iTunes; the books are listed in the right pane.
4) Drag the books from there onto the “iPad” entry in the “Devices” group.