I find what I am looking for in good training books and videos has changed as I have become more experienced. As a beginner trainer I highly valued , well, anything I could get my hands on! Terri Arnold's set of books were my first, and I used them as my foundation of training for several years.
As I became more experienced and had been exposed to more training, I found that things I would have loved as a beginner seemed boring to me now, because it was pretty much the same information no matter where it was coming from. I remember being so excited the first time I watched an Anne Marie Silverton training tape. And I remember a few years later watching another one of her tapes and being so bored I fell asleep during it.
Now what I am looking for, instead of just a good general training guide, is someone who is going to be presenting unique ideas I hadn't heard elsewhere before. Like when Connie Cleveland on her DVD suggested making a pipecleaner handle for little dogs so you have something high to grab onto when doing off leash work. As a previous trainer of a small dog, I will say: Brilliant!
I think the item that had the most unique ideas lately for me was Sandy Ladwig's DVD's. She had a lot of training techniques I hadn't heard before but made a lot of sense to me.
I remember when I used to have a problem come up in training, I would get out every training book I owned, open them all up to that section in the book, and see which book offered something that I thought might work for my particular problem. Now when I have a training problem come up I just go to one of my many online training groups and ask for advice.