New answers tagged wooden-furniture
Extend the "free post" upwards, and you can run diagonal (or bookcase) braces on the top. Alternatively, fasten it to the floor - with the frame fastened to the walls, fastening the bottom of the post to the floor and the top to the frame will mean it can't move. I guess you didn't actually say you'd be attaching it to the walls, but you should consider ...
This appears to be a "sawtooth shelf support" like the one shown in this article.
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