In each subsequent book, the clever, trap-eluding games part has diminished, until it barely exists towards the end of Mockingjay. What's left is bleak and unromantic, but generally thought-provoking. Two-thirds of the book is really a roll call of all the people who've died or are going to die on behalf of the rebel cause. The only bright spot is provided by Buttercup, Prim's indomitable cat.
There's not much left of heroic, thinks-well-on-her-feet Katniss Everdeen, our heroine of The Hunger Games. She's physically and mentally damaged from all that has happened to her. In fact, she's nothing but a symbol in Spandex, hoisted away by rebel handlers at the slightest whiff of danger. She trades more active involvement in the rebellion for the lives of her family and few friends -- and the right to kill President Snow when the time comes. However, it turns out she can't help her friends or herself very well.
There are still clever ploys and heroic actions to move the plot along, but this was definitely the most somber of the books. We are meant to think about the cost of both compliance and rebellion, how much individuality to trade for the common good, how we should never turn a blank face to suffering, and what loss is acceptable for a greater good. We are meant to see that small acts are sometimes as great as big actions, that the worst behavior is no action at all.
I'll leave plot specifics to other reviewers because you deserve to be as surprised at the plot twists as I was. Collins has created an unusual book, one whose ambiguous morality calls for mature readers. Years ago, The Lord of the Flies engendered the sort of discussion that I'm sure Collins' trilogy will inspire.