What Is The Climax Of The Book Narnia?

The first answerer is correct, but I have a feeling you may be talking only about the first book of the seven, The Lion, The Witch and The Wardrobe.  If you are, the climax would be when Aslan returns from the dead (although he really wasn't) and rescues the children from the White Witch.

Narnia is a set of seven books, not a single book. The "climax" is in the final book, The Last Battle.

There are seven books in the Narnia series, each one a complete story in itself.  The first one to be written was "The Lion, the Witch and the Wardrobe", but the books were not written in the order of the series' time line.  C S Lewis himself said that the correct order to read them in was as follows:-

The Magician's Nephew

In this book Narnia is founded by Aslan and all the talking beasts are created.  In this book Professor Kirke is a boy, Diggory, who through his own wrongdoing inadvertently brings evil, in the form of the white witch, Jadis, into Narnia.  Aslan forgives him and allows him to take back to his own world a magic Narnian apple to cure his dying mother.  This book ends with his mother being cured and he plants the apple core into the ground, where it grows into an ordinary apple tree.

The Lion the Witch and the Wardrobe

Professor Kirke is now an old man.  He lives in a country mansion but, before he left his boyhood home, he had the apple tree cut down and made into a wardrobe which he keeps in one of his upstairs rooms.  The four Pevensie children, Peter, Susan, Edmund and Lucy are evacuated to his house during the war, and find their way into Narnia through the wardrobe.  While there they assist Aslan in defeating the White Witch and become Kings and Queens of Narnia together.  They rule for many years during this visit.  The book ends with them, as grown-ups, accidentally stumbling back into the wardrobe while on a hunting trip.  They all tumble out onto the floor to find themselves children again with no earth time elapsed at all since they went into the wardrobe.

The Horse and His Boy

This is about a Calormene peasant boy called Shasta who is actually the long lost heir to the throne of Archenland, the country between Calormen and Narnia.  While making his escape from Calormen with an aristocratic girl called Aravis, who is fleeing an unwelcome arranged marriage, he finds himself virtually kidnapped by King Edmund and Queen Susan, who are on a state visit to Calormen and have mistaken him for his twin brother, Prince Corin, who had naughtily run off while he was supposed to be with them.  Prince Cor (Shasta) saves Archenland and Narnia by running ahead to warn King Lune of Archenland that he is about to be attacked by Calormen.  The book ends with the Calormenes defeated and King Lune recognising his long-lost son.  We are told that Cor is sent to be a squire in the household of the Kings and Queens of Narnia for royal training and that he eventually becomes a very good king of Archenland and that Aravis becomes his queen.

Prince Caspian

One year  after the Pevensies first visit to Narnia, they suddenly find themselves back again.  Hundreds of Narnian years have elapsed and a new race of men called Telmarines now rule Narnia.  The dwarfs, centaurs, fauns and talking animals of Narnia are all in hiding deep in the forest.  Prince Caspian is a teenager, who is the rightful Telmarine heir to the throne of Narnia, which has been usurped by his wicked uncle, Miraz.  This time the children under the authority of Aslan, help Prince Caspian to defeat Miraz and become King of Narnia.  This book ends with Aslan sending the Pevensies back to their own world through a magical doorway.  He also tells Peter and Susan that they are now too old to come back again.  The same magical doorway transports the Telmarines who do not wish to accept the new rule back to the island kingdom from which they originally came.

The Voyage of the Dawn Treader

Edmund and Lucy, together with their obnoxious younger cousin, Eustace, suddenly find themselves on board the Dawn Treader, Prince Caspian's royal ship.  Aslan has sent them to help Caspian find seven lords of Narnia who, because loyal to himself, had been banished by Miraz.  They sail ever westward, finding three of the seven.  The last stop is an island ruled by a retired star.  There they find the last three lords in an enchanted sleep.  To break this enchantment they have to sail to the utmost west and one or their number must be left behind to go on into Aslan's country.  This book ends with Reepicheep, the talking mouse, sailing into Aslan's country in his little coracle.  The Pevensies are also sent back to England.  Aslan tells Edmund and Lucy that they are now too old to return.  Meanwhile, Caspian returns to Narnia with the four of the five lords who are still alive, and the daughter of the retired star, who becomes his queen.

The Silver Chair

Eustace, much improved as a result of his first visit to Narnia, is summoned back a few weeks later, together with his friend, Jill.  Again, years have elapsed in Narnia, for Caspian is a very old man.  Aslan has summoned them to rescue Caspian's son, Prince Rillian, from a wicked witch who has kept him in captivity for twelve years.  This they do with the assistance of the Marsh-Wiggle, Puddleglum.  This book ends with Prince Rillian being returned home just in time to say "Goodbye" to his dying father and Eustace and Jill are returned to their school, where Aslan deals very effectively with the bullies who had been tormenting them.

The Last Battle

Eustace and Jill are again summoned to Narnia to help Prince Tirian re-establish both his throne and the true worship of Aslan.  All the talking animals are being deceived by an ape called Shift, who is passing off a donkey disguised as a lion as the true Aslan, and is using this deception to sell all of Narnia into slavery to Calormen.  This time, when the battle is over, instead of sending the children back, Aslan wraps up the world of Narnia, which is swamped in complete darkness.  The book ends with Aslan telling the children to "go further up and further in" to a beautiful country before them which is very like the old Narnia.  Here they meet with Peter, Edmund, Lucy, Professor Kirke and others.  They also see Mr and Mrs Pevensie at a distance and realise that the train they were all riding on when summoned by Aslan had, in fact,  met with an accident, so they are all dead and actually in heaven, the "real Narnia", and will never have to leave it again.  

This is the ending to the whole series.  Incidentally, at the very end of the book, Aslan transforms before them into Jesus Christ.  C S Lewis wanted to make absolutely sure all his reader know what the imagery of the lion, Aslan, in his books stands for.