This page includes links to recommended resources for learning Grasshopper.
Because Grasshopper runs within Rhino, a basic knowledge of Rhino commands and procedures is essential for the beginning Grasshopper student.
AAD_ALGORITHMS-AIDED DESIGN, by Arturo Tedeschi. The book covers Grasshopper in a thorough, detailed fashion, with many example definitions to try.
David Rutten, the creator of Grasshopper, has several tutorials online. Here are two introductory ones:
Mode Lab has an excellent series of Grasshopper tutorials: http://tinyurl.com/z6nubjw
Digital Toolbox has a collection of tutorials organized by experience level. Although some of the tutorials are designed around older releases of Grasshopper and thus may require some interpretation in order to implement, they give a good feel for how the software works. Here are the links: