The Lamentations of Jeremaiah is kind of an extension of the Book of Jeremaiah. It's like the epilogue, in five chapters.
After Jerusalem and the holy temple are destroyed in the 6th Century, BCE, Jeremaiah takes off to a cave outside Damascus and writes this poetic lament for the city and its people. "My sighs are many, and my heart is faint." -- Lamentations 1:22
Damn those Babylonians. Damn them!
But then, there is hope, in the least likely of places: "This I recall to my mind, therefore have I hope. It is of the Lord's mercies that we are not consumed, because his compassions fail not. They are new every morning: great is thy faithfulness. The Lord is my portion, saith my soul; therefore will I hope in him." -- Lamentations 3:21-24
I wasn't expecting Jeremaiah to offer this kind of message. But then again, he ends with this:
"The joy of our heart is ceased; our dance is turned into mourning. The crown is fallen from our head: woe unto us, that we have sinned! For this our heart is faint; for these things our eyes are dim." -- Lamentations 5:15-17
I think Lamentations can be summed up thus: God is very wroth. He will show mercy... but not yet.