I actually have tried this. I own the iron mind grippers and can successfully close the second one 3-4 reps. But ultimately I have pain that comes back when I start to hold a book, especially a large one.
If you can only do 3-4 reps you need to go to the level below that one. You also need to build forearm strength, which you can do with a dumbbell and forearm curls. Your goal is endurance, so many reps at a lower weight. Bicep curls wouldn't be a bad idea, either.
Good point! Sometimes the most obvious things just need to be pointed out for me to be like, eureka!