Not sure if anyone is still checking this thread, but I just wanted to reiterate what a number of users have already commented - the cyanogenmod 9 (CM9) install is the way to go if you have an HP Touchpad.
Let me tell you, I spent HOURS trying to sideload books so that they'd work with the webOS version of Kindle. Even when I got the process down to a science, I'd still run into issues with covers not loading properly. The CM9 install allows you to run Android's version of Kindle. Once installed, you can download "Send to Kindle" from Amazon and send books to your device with a right click. This version of Kindle also works much better with Calibre.
Again, this is old news by now. But in case any latecomers stumble upon this thread, read through the earlier replies for links to install CM9.