When you place items on the book, their weight is pushing down on the eggs. You might think this would make the eggs crack straight away, but eggs are actually surprisingly strong. The force of the push from the items is actually spread all around the egg's surface, meaning that it doesn't crack - hopefully.
Also, here we are using 4 eggs, so each egg is only taking ¼ of the weight of the items, which is why you can put more items on the 'eggy stool' than you might have expected.
Make sure that all the objects you use to place on the book, you can lift easily without hurting yourself.
Remember that this experiment could get messy if the eggs crack. So do this somewhere safe and do not do this on a glass table.