The main documentation is located at https://docs.cython.org/. Some recent features might not have documentation written yet, in such cases some notes can usually be found in the form of a Cython Enhancement Proposal (CEP) on https://github.com/cython/cython/wiki/enhancements.
[Seljebotn09] contains more information about Cython and NumPy arrays. If you intend to use Cython code in a multi-threaded setting, it is essential to read up on Cython’s features for managing the Global Interpreter Lock (the GIL). The same paper contains an explanation of the GIL, and the main documentation explains the Cython features for managing it.
Finally, don’t hesitate to ask questions (or post reports on successes!) on the Cython users mailing list [UserList]. The Cython developer mailing list, [DevList], is also open to everybody, but focuses on core development issues. Feel free to use it to report a clear bug, to ask for guidance if you have time to spare to develop Cython, or if you have suggestions for future development.
Cython developer mailing list: https://mail.python.org/mailman/listinfo/cython-devel
D. S. Seljebotn, Fast numerical computations with Cython, Proceedings of the 8th Python in Science Conference, 2009.
Cython users mailing list: https://groups.google.com/group/cython-users