What is the main theme of the book Of Mice and Men and how does the author show it?

Certainly debate will exist over what the "main" theme is
of a work of literature. However, one theme you can definitely identify and talk about
is the use and abuse of power in this book.

One
relationship you will definitely want to examine is that of the boss and his workers.
The first time the boss meets George and Lennie it is clear he has power over his
workers, as they need employment to survive, and he has the ability to hire and fire at
will. We can see this in his suspicion about Lennie and why he doesn't answer any
questions and he checks very carefully why they finished their last job. He then says to
George and Lennie: "'All right. But don't try to put nothing over, 'cause you can't get
away with nothing. I seeen wise guys before...'" Clearly the future of the workers lies
in the boss' hands - he will not be messed around, and if George and Lennie don't work
hard or cause any problems they will be fired.

Curley feels
that he has power over the men at the farm due to his status as the son of the boss.
This is why he struts around importantly, as his word to his father could result in one
of the workers getting fired. He is full of self-importance and we can see in his fight
with Lennie (although of course he loses it) that he feels he has more power than the
workers. He says to Lennie: "'Come on, ya big bastard. Get up on your feet. No big
son-of-a-bitch is gonna laugh at me. I'll show ya who's yella." This shows he does not
expect any oposition and takes his power for granted.

You
also have to examine the relationship between George and Lennie, because George
definitely has power over Lennie, though he, perhaps unlike others, uses his power for
good, to help Lennie and eventually to save him from a nasty end. George says about
Lennie that "he could have broke every bone in my body but he never lay a hand on me",
thus showing although Lennie is so much more physically powerful than George, it is
George who has the power over Lennie, and it is to George that Lennie looks when he
needs comfort and advice.

Lastly, Curley's wife has power
over Lennie in her ability to manipulate and persuade him. Although George has told
Lennie not to talk to her, she persuades him to stay and talk anyway: "'What kinda harm
am I doin' to you?". Curley's wife obviously misuses her power for her own ends,
although she doesn't realise the folly of spending time with Lennie until it is too
late.

Hopefully this will give you enough to talk about in
your work! Good luck!