Along with the iTunes 9.2.1 update, Apple released iBooks 1.1.1, bringing audio and video content functionality for iBooks.The update also includes improved PDF support and other minor bug fixes.

The iBooks 1.1.1 update includes:
? Double-tap an image within a book to view it in greater detail
.?? Experience books that include audio and video.?
? Enjoy substantial performance improvements when reading PDFs.
?? Look-up definitions to English words inside books without a specified language.?
? Addresses an issue that may have caused some book downloads to not complete.
?? Includes many stability and performance improvements.

You can find iBooks here.