I recommend this book if you won't be bothered by it's graphic nature.
Jack and the Man in Black use Desmond to temporarily extinguish the light at the heart of the island. The Man in Black makes himself mortal again and almost kills Jack but Jack and Kate kill him first. Realizing his destiny, Jack sacrifices himself to relight the heart of the island, so his friends can escape on the Ajira plane. Hurley takes Jack's place as protector of the island and makes Ben the new advisor. The flash sideways is revealed to be a type of limbo, or holding pattern, for the main characters, where they reside until accepting their life and death, and are then able to "let go" and "move on" together.
The survivors split into two groups; one led by Sawyer, the other by the Man in Black. Sawyer's group travels to Hydra Island, where they are captured by Widmore's crew after Sun and Jin are reunited. In the flash sideways, the passengers of Oceanic 815 continue to cross paths with each other.
After the remaining survivors are reunited, Sawyer and Jack hatch a plan to divert the Man in Black's attention and leave the island without him on Widmore's submarine, but disastrous consequences await them. In the flash sideways, Jack investigates the cause of Locke's paralysis and offers treatment.
"Across the Sea"
Jacob and the Man in Black are revealed to be twin brothers. They are raised by a mysterious woman who is charged with protecting the island. As revenge for the murder of their adoptive mother (who killed their real mother), Jacob throws his brother into the source of electromagnetism after being made the island's guardian, which transforms the Man in Black into the smoke monster.
"What They Died For"
Ben finally takes his revenge on Widmore after accepting the Man in Black's request for assistance. Jacob explains the purpose of the candidates, and Jack volunteers to take his place. The search for Desmond reveals that he has been rescued and the Man in Black plans to use him to destroy the island. In the flash sideways, each person ends up on their way to a concert.
Richard Alpert's back story is shown: In 1867, he is brought to the island on the Black Rock as a slave. He makes a deal with Jacob to be his representative in exchange for immortality. In the present day, Hurley convinces him to help the candidates replace Jacob and stop the Man in Black from leaving the Island.
After Jin is kidnapped and taken to Hydra Island by Widmore's team, the Man in Black confronts Widmore. On the main island, Sun loses her ability to speak English. In the flash sideways, Sun and Jin are abducted after failing to complete a transaction for Sun's father.
"Happily Ever After"
On the island, Widmore subjects Desmond to a large amount of electromagnetic energy as an experiment. In the flash sideways, Desmond, with the aid of Charlie, starts to have visions of his life in the original timeline.
"Everybody Loves Hugo"
In order to prevent more loss of life, Hurley leads the other survivors to speak with the Man in Black. In the flash sideways, Hurley meets Libby, while Desmond continues his mission to let the Oceanic 815 passengers know of their lives on the island.
On Jacob's orders, Hurley leads Jack to a lighthouse. Meanwhile, Claire questions an Other she has captured while tending to an injured Jin. In the flash sideways, Jack is a single father trying to bond with his son.
After recruiting Sayid to his cause, the Man in Black issues an ultimatum to the Others: either join him or die. Meanwhile, Kate is reunited with Claire. In the flash sideways, Sayid helps his brother, who is in debt to a loan shark.
Ilana discovers Ben killed Jacob, so she plans to kill him. Meanwhile, Jack, Hurley and a suicidal Richard visit the Black Rock. In the flash sideways, Ben, a high school teacher, tries to blackmail the school's principal.
The Man in Black sends Sawyer on a reconnaissance mission to Hydra Island, where he discovers that Charles Widmore has returned to the island with a team of scientists. In the flash sideways, Sawyer is a lonely cop who is still searching for the original Sawyer.