OpenCV's documentation can be found online at http://docs.opencv.org/. The documentation includes a combined API reference for OpenCV's new C++ API, its new Python API (which is based on the C++ API), old C API, and its old Python API (which is based on the C API). When looking up a class or function, be sure to read the section about the new Python API (the cv2 module), and not the old Python API (the cv module).
The documentation is also available as several downloadable PDF files:
API reference: This documentation can be found at http://docs.opencv.org/modules/refman.html
Tutorials: These documents can be found at http://docs.opencv.org/doc/tutorials/tutorials.html (these tutorials use the C++ code; for a Python port of the tutorials' code, see the repository of Abid Rahman K. at http://goo.gl/EPsD1)
If you write code on airplanes or other places without Internet access, you will definitely want to keep offline copies of the documentation.
If the documentation does not seem to answer your questions, try talking to the OpenCV community. Here are some sites where you will find helpful people:
The OpenCV forum: http://www.answers.opencv.org/questions/
David Millán Escrivá's blog (one of this book's reviewers): http://blog.damiles.com/
Abid Rahman K.'s blog (one of this book's reviewers): http://www.opencvpython.blogspot.com/
Adrian Rosebrock's website (one of this book's reviewers): http://www.pyimagesearch.com/
Joe Minichino's website for this book (author of this book): http://techfort.github.io/pycv/
Joe Howse's website for this book (author of the first edition of this book): http://nummist.com/opencv/
Lastly, if you are an advanced user who wants to try new features, bug fixes, and sample scripts from the latest (unstable) OpenCV source code, have a look at the project's repository at https://github.com/Itseez/opencv/.