This free ebook, named Volume 1 of Anatomy of the Face: Sculpting the Skull, is a part of new series of Ryan Kingslien. Whole series is focused on instructional anatomy references. This free .pdf tutorial show us step by step how to create an accurate base skull and also explains the proportions of the skull.
It is good way how to start to learn ZBrush.
More about this and download link, you will find here: Pixologic