Eclipse is the third book in the Twilight Saga. It continues the story between Bella, Edward and Jacob. The relationships between the three character are continuing to grow and change.
*spoilers to follow*
Jacob is in love with Bella, Edward is in love with Bella and in Eclipse, Bella realizes that she loves both Edward and Jacob. She also realizes that she can in no possible way live with out Edward so in turn must break Jacobs heart so that she can be with Edward. She also has an obligation to the Cullen family that has carried over from book two, "New Moon". She must become a Vampire before too long or she risks putting the Cullen family in danger.
On top of the little love triangle, Victoria is back and is still seeking revenge for the death of her beloved James from the first book "Twilight". Only this time she creates an army of "newborn" vampires to help her. There are not enough members of the Cullen family to defeat them all so the impossible happens and the Cullens seek the help of their enemies, The Werewolves. Jacob helps to convince his fellow Werewolves to help and they do.
The story continues in the fourth and last book of this Saga, Breaking Dawn.