But, you can't delete a book if the device reboots before you can get to the library menu or connect to USB (unless you open the device to remove the internal SD card, but this is not a good idea on the H2O because the waterproofing seal will be broken.)
You should be able to initiate a factory reset (not just a basic or pinhole reset) somehow,
this page has one method (manual factory reset), and I think there are others if that doesn't work, but you might want to wait to hear from someone who has done it on a H2O.
Edit: Someone had a similar problem in
this thread, there are some details there about the different ways to start a manual factory reset.