I need a good thesis for the Inverted World novel. If it's possible, an easy one.

Writing a "good and easy" thesis is tough. Thesis writing in general is tough because of how critical of a single sentence a thesis is. A thesis statement guides everything that comes after it. Additionally, it needs to make an argument. It can't be a statement of fact, because there would be nothing to prove.
I think a relatively easy and consistent thesis format is the two-part thesis statement. This involves an argument and a counter-argument, and it begins with the word "although." Starting with that word guarantees that the statement leads with a dependent clause that then needs to be finished with the other half of the argument.
For the story Inverted World, you could write a thesis that examines certain themes of the novel. I would recommend something about the tight social class structure that the story has and how that affects the flow of information between people and classes. For example, "Although the story's social class divisions are supposed to make the city work more efficiently, they are actually detrimental to a cohesive society."