Lamentations is a book of five poems, each a chapter and the first four of which are acrostics (a typical form of poetry in Hebrew — see also Psalms 37, 119, and 145, and Proverbs 31:10-31). The author is Jeremiah: Having unsuccessfully warned his people in the book of Jeremiah to turn back to God and away from sin, he now writes to bemoan what has befallen them instead, namely the fall of Jerusalem to the Babylonians in 586 B.C. (the author is disputed, but not this historical context for the book).
This is by far the shortest of the books by major prophets, and its message is straightforward: God can bring serious grief to those who sin, and indeed may do so even if it results in the (temporary) triumph of those He likes even less; when this happens, we should respond with sorrow and contrition; the good news is that, with God’s great compassion and love, there is always hope for those praying to Him for His help.