There is a children's book published in the early 1980s which was about a pair of islands. On one island the people were rich and valued money above all else, and on the other island the people were not.
The island with the rich people began to sink beneath the ocean as a result. The cover had a beautiful illustration of the two islands with a third sunken island clearly visible in the illustration presumably indicating that it had happened before.
My memory is of a hardback book perhaps 3mm thick and about 1ft square in size.