Tall bookcase, floor out of level – what would you do?

[Bill Dufour]

If you live in North America you live in either/or earthquake or hurricane country. Either one will knock over a bookcase onto your head.
bill D

[Tom Bender]

Another thing that can help is making it 3 footed. Adjustable feet under the front edge and only one in the back at the center.

[Vince Shriver]

+ 1 with Bill. Here on the left coast you can sometimes "feel the earth move under your feet". Always prudent to tether certain furniture to the wall.

[John K Jordan]

...I have not felt the need to add trim to hide the wedges, but I am an informal person. ...

Ah, I see that as an indicator of balance, key to a happy and satisfied life!

[glenn bradley]

Another vote for wall attachment from one who lives where the ground sometimes does the Watusi. Level the unit, attach to the wall, and scribe in some trim.