Steinbeck picked the name of this book from a poem line:
The best-laid plans of mice and men often go awry
Which is quite fitting once you finish reading "Of Mice and Men". Frankly, it's obvious from the beginning that the book is going to end badly. In some way the conclusion is inevitable - it just couldn't end differently.
So much has been written about this short book (~100 pages, just a couple of hours of reading), so I won't get into long narratives. I've just skimmed through a few reviews and analyses and don't feel like repeating what others have said countless times.
The bottom line - the book is great, highly recommended. There's so much human nature in those few pages, it's just amazing how Steinbeck managed to pack it all in so nicely. Many of the most major and fundamental principles of the human psyche are here - self-esteem, meaning to one's life, loneliness, friendship, love. Any way you look at it, the book is a masterpiece. Surely worth reading, more than once.