I am forever looking for the prefect reader (epub) on the iPad. Apple’s iBooks app is fine; it could do with a line spacing option though.
Here are 2 ebook readers that are quite good in their own rights: Readmill and Marvin. Marvin is almost perfect for me: I love the Gentium font and the ability to connect to Dropbox. The only fault is that you can’t control the top and bottom margins. Readmill also uses a great font – FF Scala. Now, if only they give a font size option between what is currently small and medium, and have the app have the ability to justify non-justified formatted epub files, this app will be the prefect one for me.