ᐅ How can I increase the stability of additional shelves in BILLY bookcases?

Hello everyone, I have a few BILLY shelves from IKEA at home and want to reinforce the shelves because I mainly want to stack heavy books on them. My question is: How can I add extra stability to BILLY shelves without significantly changing the appearance or making the assembly more complicated? Are there any specific tricks or materials that work well to prevent sagging? I look forward to your experiences and tips!