Version 3.1.3 has been released in the App Store:
What’s New in Version 3.1.3
• Updated and enhanced for iPad’s new Retina display – words and images load sharper
• Easier-to-use Highlight feature for NOOK Books – highlight all the words you want
• On NOOK for iPad only: Now read in 1 or 2 columns when using landscape orientation
• Several bug fixesThis update requires iOS 4.3+ and works on the new iPad, iPad2, iPad1, iPhone 4S/4/3GS and iPod touch (3rd generation and above).