Then he was flying once more, unrestricted in the blue-black expanse of the sky. The stars blazed timelessly around his head, and their patterns became known to him. They were both similar and different from the shapes and powers that men of long ago had attributed to them. The Herdsman passed by, nodding, with the bright star Arcturus at his knee. The Bull roared past, carrying the great sun Aldebaran and the small group of the Pleiades, singing in soft, melodic voices like no others he had ever heard. Up he flew, and outward, through the black void of space. He saw the dead stars, the blazing stars, and the thin scattering of life that populated the infinite emptiness beyond. And when he was done... he knew the mystery of Uranus and the despair of Mercury, and he had ridden on a comet's tail. [p. 105]
I have a deep affection for this series. As a child, it was a source of great joy and excitement for me. Even as an adult, I still find myself drawn to it, although I must admit that the last book left a lot to be desired. When I was younger, I didn't have such strong opinions, but as I've grown older, I've noticed some major flaws in it. I would highly recommend reading the first four books and then checking out the Wikipedia entry for the fifth. This way, you'll have a good understanding of the series without having to endure the disappointment of the final book.
The links below are to my reviews of each book. I like to keep all of this information in one place for easy reference. Here are my ratings:
Over Sea, Under Stone: ★★★★✰
The Dark Is Rising: ★★★★★
Greenwitch: ★★★★✰
The Grey King: ★★★★★
Silver on the Tree: ★★✰✰✰