The book to read is "In Bad Company: America's Terrorist Underground" by criminologist Mark S. Hamm. It offers a more compelling "why" behind the OKC Bombing than any other book and these FBI docoments being sought directly back up Hamm's work.
I'll have to check that book out. The third terrorist simply paints a who what where when and how from a journalist perspective, but it doesn't touch much on the WHY behind the attacks.