Thanks very much for the detailed reply! Excellent guide on how to do it.

The assembly is pretty straighforward, they didn't really mess that up.
These guys simply didn't have experience in doing this. They didn't check
if it was level, just shoved it against the wall and put some wood under a
corner to keep it from leaning over. That's maybe good enough if your
bookcase is on 4 feet, but with this construction it doesn't work so it
seems.

Thanks for all the clarification and help. I'm more confindent I can fix
this now

I've asked a co-worker with lots of muscle to come and help :-)

Have a merry Xmas!

Neko