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.
It was good advice about 20 years ago … hell, I remember furnishing my first home with a used mattress back then. I remember I had to daily take it out and place in the sun and wind for a week every day, flipping it every few hours … to get rid of the horrible cigarette smell from someone who was a chain smoker!
It’s not a good idea any more because of the danger of bed bugs.