The Lord of the Rings by J. R. R. Tolkien
The Lord of the Rings is an epic fantasy novel published in the 1950s. It was published in three volumes and is often (understandably) mistaken for a trilogy. It is one continuous story and is available nowadays as a single volume.
The story follows the adventures of Frodo Baggins, nephew to Bilbo Baggins, the title character from The Hobbit. In that book, Bilbo found a strange magic ring that made him invisible. In this book, he reluctantly passes on this treasure to Frodo. They are both friends with Gandalf, a wizard who passes through their home town of Hobbiton every now and again. Gandalf suspects the ring has more to it than the hobbits know; through research he discovers it might be the one ring forged by Lord Sauron in Mount Doom to give him power over the other races of Middle Earth. The ring was lost for hundreds of years after Sauron lost a battle to an alliance of men and elves. Something must be done with the ring now, so Gandalf sends Frodo on an epic adventure to dispose of the ring.
The novel is an amazing work of world-building. Middle Earth is a vast expanse populated by a wide variety of races who all have distinct histories, qualities, and languages. The history of the land is long and bits of that history keep coming out as the adventurers continue their journey. Middle Earth is a fully-realized other world.
The moral imagining of the world is also impressive. Though God is never mentioned, the characters (especially Gandalf) often note the providential nature of events. When Frodo arrives in Bree to meet Gandalf, the wizard is not there. In stead, Frodo meets Strider, a northern ranger, who helps them on their way. Gandalf had left a note to be forwarded on to Frodo instructing him to hurry along but it was never delivered. Gandalf is kidnapped right after leaving the note. If it had been delivered, Frodo would not have found Gandalf or Strider. It's a case of dumb luck or divine providence. The characters acknowledge providence even if they never mention God.
The morality in the book is deeper and more complicated than it is often given credit for. Characters struggle to choose the right thing to do and sometimes they choose wrongly because of their own personal flaws or doubts. Boromir, one of the fellow travelers with Frodo, tries to take the ring to use it as a weapon against Sauron. That choice causes a lot of immediate problems (including dividing the group just as a band of orcs attacks). He realizes his mistake and valiantly dies fighting the orcs. He has a moment of redemption. Few characters are irredeemably evil or perfectly competent.
The Lord of the Rings is a triumph of narrative fiction and is, in my opinion, the greatest novel ever written.
For more commentary on the book, check out A Good Story is Hard to Find's discussion, Part One and Part Two.