Can you post a picture so we can see?

If this is a flat pack, it sounds like you don't have your bookcase squared up. You might need an extra helper to hold it perfectly straight and square while you tack the backer in place. You should not end up with a warp at all, that's just indicative that the case itself isn't squared up.

Good luck :)

Hi,

you might need to post some pictures in order to get help.

But here are a few questions for you in the meantime.

Is it sitting on level floor? Check with a level. It could be making it out of square.

Have you assembled the rest of the bookcase? Are you sure you followed the instructions using all the right hardware in all the right places?

Is it sitting square in all corners. If it is leaning or one corner is out, then it is going to throw the fit of the back out completely. If you don't have a carpenter's square, borrow one and check. Once it is square, if the back piece still doesn't fit, then I'd speak to the manufacturer about it before trimming it.

It is possible it is the wrong size but if you cut it and ruin it, then you're going to have a bigger mess on your hands.

thanks

Is this from IKEA? If so recheck the assembly instructions.

Because they make so many of the same kind of items it's more likely operator error instead of manufacturer error.

Lay the book case down on brown paper, trace a correct proportion, and head to Home depot. Cut plywood to correct size, and paint. Otherwise, my guess is you have some imported junk on your hands and thus call the source and ask for a return label. With luck, they tell you to junk it.