Susan Cooper's The Dark is Rising Sequence consists of five books as follows:
The first book, Over Sea, Under Stone, features the three Drew children: Simon, Jane and Barney, who are staying in Cornwall with their parents and their "Great Uncle" Merriman Lyon. In the attic of the house in which they're staying (rented from a sea captain), they find an ancient map which leads them on a treasure hunt to find a long-hidden Grail that once belonged to King Arthur. But the three of them have to figure out the clues on the map (it's not a straightforward "X marks the spot" treasure map) and Merriman cannot help them much as he needs to keep three rather sinister people from discovering that the children have the map, and then keep them from gaining the map and finding the Grail for themselves.
The second book, The Dark is Rising, features a different boy, Will Stanton, who on his 11th birthday, meets the mysterious Merriman, who tells him that he is the last of the immortal "Old Ones", a circle of people throughout history and spread across the world, who collectively are fighting the forces of evil (known as the Dark). As the power of the Dark grows, Will has to travel backwards and forwards in time (something an Old One can easily do) in order to recover the six Signs of Iron, Bronze, Wood, Fire, Light and Stone, which can help to stop the Dark from overcoming the Light, and protect his family from the malice of the Dark.
In the third book, Greenwitch, the three Drew children are visiting Cornwall again, staying in a cottage with Merriman and next door are Will and his American uncle and aunt. The grail has been stolen from the museum where the three Drews had deposited it and they four are in search of it because it, like the Signs, is a Thing of Power for the Light and is important in the final fight against the Dark. However, Jane is haunted by nightmares about the Greenwitch, a symbolic image woven from branches and leaves that is annually cast into the sea in the hopes of ensuring a fruitful year. Barney, meanwhile, is captured by a sinister painter, who wishes to use the Grail and Barney to foresee the future. But the Greenwitch is not just a symbolic image, it's alive with Wild Magic that neither Old Ones nor the Dark can control, and it holds a manuscript which holds the key to translating a runic inscription on the Grail. Somehow, the children and Merriman must persuade the Greenwitch to give up its "treasure" to them, rather than to the painter who is the Dark's representative.
In the fourth book, Grey King, Will is suffering from a bad illness and after he passes the danger zone, he is sent to recuperate in Wales, with his mother's old friend and her husband. On Clwyd Farm, Will meets Bran Davies, an albino Welsh boy whom he names the "raven boy", and a dog with silver "eyes that see the wind" - both are part of an old legend, and are mentioned in the old rhyme that Will learnt from the runic inscription on the grail. Will leads Bran into discovering his history and past, although the Dark does all it can to prevent Bran from joining Will and the Old Ones.
The final book, Silver on the Tree, brings the series to a climax. Will and Bran journey to the Lost Land to recover the final Thing of Power, the Sword Eirias. They must face many challenges, any one of which could destroy them, before they can gain the sword - but once they have it, the final battle with the Dark is yet to come.
Fantasy incorporating legends and old myths is nothing new, but Susan Cooper brings the idea of time-travelling immortals, ancient magic, adventure and tragedy together in an interesting and enjoyable series of books. Cooper's writing is atmospheric and full of detailed descriptions. Whilst some readers may not like the fact that good and evil are portrayed as evenly matched, the strength of the Old Ones' determination is very invigorating. The Old Ones are powerful, but they are still very human. And the "lessons" which Cooper includes in the series are carefully interwoven; these lessons about good and evil, about compassion, loyalty, friendship and redemption are never overtly moralising. My favourite character is Will Stanton, who is a mixture of a wise ancient being and a pre-teen boy.