To be honest, I started neglecting the core work after my hamstring pain stopped. However, I still occasionally get a sharp, yet mild pain in my back after extending it, e.g. during OHP and getting up after benching. I've even had it after lying on my stomach for just a few seconds - I just couldn't straighten and flex my back, so I had to roll over to get up. Damn, I sound so geriatric! It's not always like that though and seems ok at present.
I started the curlups, planks and some back bridges the other day and feel better already for it. I've been reading the book as fast as I can because I wanted to get to the exercise section, but I didn't want to miss any of the preceeding chapters. I'm looking forward to seeing what the harder exercises are, because it's nice to have a goal in mind.
Thanks again for recommending it.