https://www.youtube.com/playlist?list=PLpoXxlr0idMkF5l8uSrdFJiWNXLE-joUn
In this free tutorial series, Isaac Oster demonstrates a variety of useful tools and techniques for generating hard surface geometry with Zbrush. It covers Zmodeler, Dynamesh, Insert Mesh Brush, Poly Groups, Live Boolean, Zremesher, Creating geometry from Alphas, working with Curves, and creating clean high poly geo with retopology.
Part 2 of this series covers the complete process of creating a hard surface model using Zbrush using many of the techniques discussed in Part 1. It is available here: https://www.youtube.com/playlist?list=PLpoXxlr0idMlwAWdx1BkTwHZfhuIdppDI