Apps and Ideas for Literature Circles on iPads -- THE Journal
This article has some ideas about how to use the iPad to support a variety of activities including lit circles. Page 2 has a list of useful apps to check out. While the article mentions iBook, Kindle and Kobo apps for purchasing books, don't forget that there are some public domain titles that might be quite appropriate: Cue for treason Treasure Island, Alice in Wonderland, etc.
SD38 also has ORCA digital titles that can be read on iPads.