/diy/ - Do It Yourself

How can I improve the stability of a bookshelf?

I just got one similar to pic except there're 5 levels, the boards are thicker, it's very heavy and there's no back (can see the wall behind it). After putting it together I realized that it moves a lot, it takes noting to make it shake dangerously and it's still empty, I worry that when I start filling it up with stuff it's probably gonna open apart.