For anyone looking to get started, I'd strongly recommend getting a trainer over a book. Chances are, especially if you've never done any sort of training, you have no idea how to properly do the basic movements like squats and presses and deadlifts. Especially deadlifts if you're doing this because you have back pain.

I would recommend a book (and YouTube) over picking a random trainer from the set of all possible trainers. I know some that are way better than a book, but I've also seen plenty that don't know how to squat or deadlift, much less teach someone else. If you're at a powerlifting or Olympic lifting gym specifically, absolutely use one of the trainers from there. Crossfit too. But if you're at a typical chain gym, I would not trust most trainers.