If memory serves me correctly, I think you have to close a Kindle book to make sure it syncs, although it can sync without closing, but to ensure it does they suggest closing the book. The iBooks app doesn't require anything. I can read to page X on my iPt4, pick up my iPad and open the book, if it isn't already, and within a few moments it will autosync and pop me to the current page. It also syncs bookmarks, highlighting, etc. Other than the syncing ability, iBooks is a fairly lackluster app as it does not allow a lot of format customization, and the app speed is slower than other reader apps, but the excellent syncing capability makes it my reader of choice. Best of all, it even syncs sideloaded books whereas Kindle and Nook only sync purchased books.