in reality things are more complicated. You'll probably have thousands of books and customers. Each book now comes, not with a pair of numbers, but with a huge long list containing the rating of each customer or perhaps a blank if a specific customer hasn't rated the book. Now you can't simply plot the data and spot the pattern.
This is where topology comes to the rescue: it gives a neat way of turning shapes into networks. Suppose you've got a wobbly circle as in the figure below. You can cover it by overlapping regions and then draw a dot on a piece of paper for each region. You then connect dots corresponding to overlapping regions by an edge. The network doesn't retain the wobbliness of the shape, that information has been lost, but its topology, the fact that it's circular, is clearly visible. And the great thing is that it doesn't matter what kind of covering you use to make your network. As long as the regions are small enough — the resolution is high enough — the network will draw out the topology of the shape.
The reason why even the most bewildered tourist can find their way around the tube network easily is that the map does away with geographical accuracy in favour of clarity. The map retains the general shape of the tube network, the way the lines connect, but it distorts the actual distances between stations and pretends that trains only run in straight lines, horizontally, vertically or inclined at 45 degree angles. That isn't how they run in reality, but it makes the map a lot easier to read. It's a topological map named after an area of maths, topology, which tries to understand objects in terms of their overall shape rather than their precise geometry. It's also known as rubber sheet geometry because you're allowed to stretch and squeeze shapes, as long as you don't tear them.