I haven't built one for a couple decades, but I used to
think it was fun to strive to build reliable shelves using material any sane person would think was too light for the task. For example, I use 3/8" plywood to make 30" wide book shelves.
I figured if engineers could come up with suspension bridges, that would hold lanes of traffic, some of those tricks could be used to build bookshelves that were so light they could be lifted and carried with one hand, and wouldn't sag under extreme load.
To be fair, I did add stiffners to the front. I rabbited the stiffner so it set in the stiffner, as well as on it.
Performance of stiffners is so impressive, I am amazed at how many nice looking kitchen cabinets don't have them, since a reasonable stack of Fiestaware results in sagging. In fact, when I met my wife, her house had some of those cabinets. I added the stiffners to the back of the shelves and they solved the sagging problem.
The shelf that comes to mind was about 7' tall. I ran some 1/8" aircraft cable [I got for a song and dance at a garage sale] in an X pattern on the back. The cables ran top to bottom and have turn buckles to allow tightening.
Stainless aircraft cable wire is fairly cheap and would allow for a lot of weight reduction, without compromising capacity.