Fairy Tail Volume 12 by Hiro Mashima
The Tower of Heaven storyline (including Erza's backstory) concludes as the Fairy Tail crew take out the remaining wizards from the Assassin's Guild and Erza takes on her former friend Jellal. Jellal's plan to raise the evil wizard Zeref is on the cusp of completion even as his twin brother Sieg finally convinces the Magic Council to use the Etherion weapon (a massive amount of destructive power) to destroy the Tower of Heaven. Can Erza finish off Jellal and escape in time? Or does she have other plans? And why isn't Jellal worried about Etherion?
The story has some interesting twists (though one big twist is very predictable) and an epic scale. Erza's story is fascinating and her growth as a person and a magical warrior is inspiring. The conclusion hits just the right note, emphasizing the importance of friendship and perseverance.
I read this before seeing the anime and after watching, they are still virtually the same--both enjoyable.