Steinbeck doesn't expect you to scale any mountains here. Sure, the dialect can be a bit tricky, but once you get used to the dropped word endings, it's pretty easy reading. Mostly, we're dealing with language like this:
Lennie dabbled his big paw in the water and wiggled his fingers so the water arose in little splashes; rings widened across the pool o the other side and came back again. Lennie watched them go. "Look, George. Look what I done." (1.9)
Our narrator is straightforward, with words like "dabbled," "wiggled," and "splashes." Our characters are even simpler, with some really questionable grammar choices. And the plot? Something along the lines of boy meets boy; boy meets girl; boy loses boy. In fact, the toughest part is making it through the darn book without reaching for a tissue.