I went shopping and found new book shelves are made with thin veneers and particle boards. So I moved away from the new and started searching for used. I discovered that there are lots of entertainment centers in charity thrift stores and used furniture consignment stores that have been given up because they are useless for the new large flat screen tv's. I picked through dozens and located this unit made with 3/4" laminate wood. Nothing in it has particle board. I measured open areas for additional shelves, then went to Home Depot and purchased a full 4x8' sheet of 3/4" cabinet grade plywood that I had them cut for me to the proper sizes. It didn't require a full sheet, but it was much more reasonable than buying smaller precut pieces. Then I finished the additional shelf boards with 3 coats of Polyshade polyurethane. There are designated shelves for the Great Atlas of the Sky and the uber size 21" Harmonia Macrocosmica. (It's beautiful, but what was I thinking? )
It all turned out very well, and fits in a walk space between 2 rooms. I made a white box shelf on top for dvd's that with shelving boards that were sitting unused for years in my garage. I'm happy with the results. It brings new life to old furniture, is better quality than what I could find new, and holds lots of books. I even have some new empty shelf space.