Recently updated for 2018! If you're looking to get into VR design, UX or storytelling, this is your first stop. It's a lean 100+page approach to the roots of a VR experience based on the current technology stack. You'll learn the basics, some advanced tips, and the problems that are still pending in the VR UX community. If you need a starting point, there are a lot of initial tips here to catch you up. Recently updated sections of the work based on review feedback (6 DOF, Google Cardboard Mechanics, etc.)
Top 50 book in Product Design in 2017
Featured on the list of VR UX resources provided by uxofvr.com