Try this book
Digital Drawing for Landscape Architecture: Contemporary Techniques and Tools for Digital Representation in Site Design https://www.amazon.com/dp/1118693183/ref=cm_sw_r_cp_api_glc_fabc_jRg9FbDSBCDF5
I believe research and using the right resources will always help you achieve your goals. Here are a few rendering tutorials that may advance your renders:
https://visualizingarchitecture.com/tutorials/ http://www.ronenbekerman.com/architectural-visualization-tutorials/ http://vyonyx.com/
http://www.amazon.com/Digital-Drawing-Landscape-Architecture-Representation/dp/1118693183
My renderings got much better after doing some of their tutorials. Although, I still think rendering is not king. If anyone has material on practicing concepts and creating narrative for your designs that would be awesome.
Cheers and good luck!