How can I fix this gap between bookcases I made?

Thanks everyone; I figured it out and came back here to update my post and noticed that Michael Karas said the same thing I realized (although it was a comment, so I couldn't mark it as the correct answer): I had to pop the back off the left unit (which was actually quite easy because it was just 1/4" plywood attached with brads and no glue), re-rack the unit into square, and then reattach the back. Thanks for the input!

Swap their positions.

Place the bookcases so that they lean into each other instead of away from each other. This will eliminate the gap between the tops.

Bookcase contents are usually quite heavy. Once the cases are filled, you will probably find that you can force the bottoms together and the weight of the contents will distort the shape of the cases so the gap disappears.

If an unsightly gap remains visible then you will need to resort to a decorative molding.

You don't have to buy or get a framing square : just measure the diagonals... That will tell you if either or both units are out of square.

If they are both ok, then look to the floor - small change in the floor will make a large gap at the top... Then you need some adjustable feet of some sort.

I have used interscrews (e.g. from screwfix) in the past to join units together to make them line up nice and tight.

If the shelves are sturdy, just get some molding and cover the junction of the two units. Nail the molding to only one unit so they can be easily separated if you want to move them.

I would turn the left bookcase upside down. If the other side of the left bookcase is more square, or slants to the right, this will fix your problem.