OpenCV has recently provided a list of books they recommend.
Other interesting materials:
There's a great introduction to OpenCV at Gady Agam's introduction to OpenCV.
Noah Kuntz OpenCV tutorials are OK. It covers image manipulation, data types, image processing, camera stuff and several other topics.
There's also the official OpenCV Tutorials, provided by the OpenCV folks themselves.
Also you can find some good Opencv C++ Examples here to get started.
