as one of my holiday books I read
The City
and the city by China Mieville.
The book was essentially a
detective story set in two cities that overlapped.
Citizens on of city were not allowd to look at
the citizens of the other city, even though they
shared the same streets. they had to unsee
things from the other city -- if the didn't
they would be "arrested" for something
called "breach".
Look it is a confusing idea, but very clever.