This is by far the best-known of the Narnia books, and the recent movie was fairly successful (deservedly so, I think).
Four British children are sent to a rambling house in the country during the blitz. They are largely left alone, and the youngest stumbles through a wardrobe into the magical land of Narnia. Eventually, all four come to know Narnia very well – but it is a whole country where it is “always Winter, and never Christmas”. The White Witch rules over all, and already has an ally in one of the children, who unwittingly (or mostly unwittingly) sets out to betray his sisters and brother.
There are fantastical creatures galore, and the White Witch is a truly dire enemy who turns animals into stone out of spite. The writing is excellent once again, the characters realistic yet heroic, and the adventures thrilling.
“This must be a simply enormous wardrobe!” thought Lucy, going still further in and pushing the soft folds of the coats aside to make room for her. Then she noticed that there was something crunching under her feet. “I wonder is that more mothballs?” she thought, stooping down to feel it with herhand. But instead of feeling the hard, smooth wood of the floor of the wardrobe, she felt something soft and powdery and extremely cold. “This is very queer,” she said, and went on a step or two further.
Next moment she found that what was rubbing against her face and hands was no longer soft fur but something hard and rough and even prickly. “Why, it is just like branches of trees!” exclaimed Lucy. And then she saw that there was a light ahead of her; not a few inches away where the back of the wardrobe ought to have been, but a long way off. Something cold and soft was falling on her. A moment later she found that she was standing in the middle of a wood at night-time with snow under her feet and snowflakes falling through the air.
Rating: PG. I’d call it absolutely G and safe for anyone, but one character is a close parallel to Jesus Christ (in one of the later books this character clearly states that he exists on Earth as well, is known by a different name there, and that the children have been brought into Narnia so that they can more easily recognise him on Earth), and some atheists have found that offensive. The books do focus on the adventures, rather than allegory about 95% of the time.