I am about to hype it to you, as well. If you're a woman, you should definitely read it. At the least, you may discover stories that resonate with your experience. If you're a man, especially if you work in management, you will come away better prepared to work with women. In general, I would describe the book as short, sensible, and useful--even if, like me, you experience ambition only as directed toward specific passions rather than as as a general drive to excel.
My eyes were certainly opened to how I deal with the workplace. Sandberg talks about how much society trains us to hold back: women often literally don't take a seat at the table in a meeting; we don't put ourselves forward when opportunities arise; we only apply to a job if we have every single skills and all the experience listed. (I had no clue that many men don't do this). Recognizing these things is not enough; it's hard work to get around these patterns, to encourage others to get around them, but it's worth it.
This hard work is what she means by leaning in.
I love that Sandberg then points out that while women need to lean in at work, men need to lean in at home. After watching friends acting as stay-at-home fathers, I can only say a giant YES to her whole section on this. The men I know are doing great jobs, but people around them shame them for it. We need to do better supporting men in the home as well as supporting women in the workplace.
In sum: read the book. Right now, if you can. The library has many copies.