So you buy a nice two shelf bookcase but it’s wobbly and you know it won’t hold much. I’ve recently gotten one and my solution was to put L-brackets on it. After installing about 8 of these brackets at the cojoining parts of the shelving, it is now completely stable and ready for use.
For younger or newer renters, I highly recommend the book Safe and Sound: A Renter-Friendly Guide to Home Repair by Mercury Stardust
It covers so many things I wish I knew when I first moved out on my own, and probably would have saved me some serious money over the years.