Shelving and flat files
If you can afford it I would invest in a flat filesystem. I have 2 that were built for me years ago by a friend. The whole thing comes apart into flat pieces that make them easy to move. They have moved with me six times over 35 years with no problems. Each flat file has five shelves, four that slide in and out and the bottom shelf which is fixed. In them I store my unused finished panels and stretched canvases from 6×8 inches to 30×40 inches.
On top of the flat files I have a wire dish rack that I purchased and use them as racks for drying smaller wet paintings. Each rack holds 8 paintings up to 12×16 in size. Larger wet paintings go on inexpensive wooden presentation easels to dry.
I like art books and have lots of them in my studio. It is easy to pick up older book shelves at second hand stores and garage sales. I look for sturdy ones made of solid wood with large enough shelves for the larger sized art books.
A taboret or small chest of drawers is a good solution for storing paints and other traditional media. My taboret is next to my palette set up which allows me to get to supplies without too much trouble.