For the free assets that give you deeper insight into how to use OpenCV with Unity, you can download them from this GitHub link: https://github.com/EnoxSoftware/OpenCVForUnity
To read the documentation on the Dlib C++ library, go to http://dlib.net/.
To learn about the methods, properties, fields, and OpenCV tutorials in C++, Python, and Javascript, and to gain deeper understanding of how OpenCV is put together, you can go to this link: https://docs.opencv.org/3.4.1/