Doesn't hold books without bowing under the weight

I bought this bookcase for my apartment. I needed it to be high quality, easy to assemble, the right size for my living room, sturdy enough to hold a lot of books, and not require a lot of money. I was willing to pay more knowing how important this piece of furniture was to me. Unfortunately, this bookcase fails the sturdiness test. The bookcase shelves bow noticeably under the weight of books. There's a reason I got an item called a bookcase... I needed it to hold heavy books. I like how this looks, and it does meet my other requirements, so I will have to make a trip to the hardware store to see what options are available my problem without giving up the whole thing.

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