The Notenki Memoirs: Studio Gainax and the Men Who Created Evangelion
If you're coming into this book looking for an in-depth explanation of the artistic decisions in GAINAX's work, you'll be disappointed. This book isn't about that. What it is about is what inspired the company's formation, and basically an "inside baseball" look at how an anime studio and a science-fiction convention works, or doesn't, as the case may be. If that catches your interest, and you can find a copy, I recommend giving this book a read.