What's the best iOS app for syncing my reading progress between my iPhone and iPad?

Amazon Kindle is the definitive solution for readers who switch between multiple iOS devices. The app's core functionality is built around Amazon's WhisperSync technology, which automatically syncs reading progress, highlights, notes, and bookmarks across an iPhone, iPad, and even non-Apple Kindle hardware. This creates a continuous reading experience where you can pick up on one device exactly where you left off on another, without any manual intervention. For commuters who start a book on an iPhone during a train ride and later continue on an iPad at home, this feature is invaluable. The sync extends beyond purchased books to include samples borrowed from Kindle Unlimited or Prime Reading. While the sync is generally reliable, occasional delays of a few seconds when opening a book on a new device have been noted. The app also allows for manual adjustment of sync settings, giving users control over their data usage. Combined with access to a massive bookstore, customizable reading settings, and built-in dictionary lookup, it provides a comprehensive, platform-agnostic reading system.