If you have an HP TouchPad and you want to use it to read books, then you should get the Kindle beta and get access to more than 950,000 titles from the Amazon Kindle Store.
Unfortunately this is only available to US TouchPad owners at present.
Here is what HP said:
The Kindle Beta for webOS app is now available for the HP TouchPad. Your TouchPad now has access to more than 950,000 titles from Amazon’s Kindle Store, as well as thousands of the most popular classic public domain titles. Paid titles offer free downloadable samples, and Amazon’s Whispersync technology automatically syncs your reading progress with other devices (like your laptop computer) where you might also use your Kindle account. (And if you add any notes, highlights, or bookmarks while you read, Whispersync also keeps those updated on your other Kindle platforms, and vice-versa.)