The New York Times: Stanza: “The iPhone or iPod Touch can act as an electronic book reader.”
Tip of the Week: Turn Your iPhone Into an e-Book

How can I link to a book on my web page so that it launches Stanza on the iPhone/iPod Touch?

Provided your book is a valid ePub or eReader file, having a link to it with the "stanza://" protocol will cause Stanza to be launched. So if your book is located at "http://somesite.com/books/mybook.epub", you just need to have an additional URL like "stanza://somesite.com/books/mybook.epub" to automatically download and import the book into Stanza's library.