This week, Apple updated its FAQ for iBooks, the iOS-based e-book ecosystem introduced with the iPad.
The updated document includes information about Collections, introduced in the latest version of iBooks, version 1.2:
Yes, you can organize your books into a bookshelf or list view. You can also organize your books into specific collections on a topic of your choosing. For example, you can put all your books and PDFs related to your work in a single collection. To use Collections, tap the Collections button, or the name of the collection on your iPhone or iPod touch to change or add new collections.