Grasshopper3d Discussion Forum
Legacy - http://www.grasshopper3d.com/forum
Current Posts - https://discourse.mcneel.com/c/grasshopper
The grasshopper community is incredibly supportive and always growing. The discussion forum should be a first port of call for any question no matter how complex. Be sure to search the forum (use google and site:grasshopper3d.com) before posting a new topic.
Rajaa Issa - Essential Mathematics for Computational Design
If it's been a while since you've worked with vector maths, this book provides an excellent summary of core concepts demonstrated through practical examples in grasshopper. Because of this format, the book is best used as a reference while following other tutorials or working your way through the Grasshopper Primer. It also covers matrices and transformations and principles of NURBs curve geometry.
Modelab - Grasshopper Primer
Modelab's excellent grasshopper primer is the first point of call for most beginners to parametric modelling. It covers the basics of the interface, geometry, algorithm design and data structures.
Exlab - Studio Air Tutorials
The Studio Air tutorials demystify parametric design process behind a series of canonical case study projects. Core concepts build in complexity from geometry and data types, to parameter space, data structures and algorithmic concepts like encapsulation and encurstion.
For a more exhaustive list of tutorial resources refer to the Grasshopper3d.com website.