For dealing with solar geometry and component population and many more!
www.tedngai.net
The definitive Intro to Grasshopper 3D by Andrew Payne and Rajaa Issa
http://www.liftarchitects.com/
Another good source of general information is by Zubin M Khabazi
http://download.mcneel.com/s3/mcneel/grasshopper/1.0/docs/en/Generative%20Algorithms.pdf
Once you get past the intro, I suggest to look at the following documents for more advanced usage.
Essential Mathematics 2nd Edition by Rajaa Issa
http://download.rhino3d.com/en/Rhino/4.0/EssentialMathematicsSecondEdition/
Connected Holes by Lars Renklint
http://blog.rhino3d.com/2009/03/connected-holes-in-grasshopper.html
Grasshopper workshops at Cornell architecture by Woo Jae Sung
http://woojsung.com/
