As this is posted in YA, I'm assuming that's approximately the age we're shooting for: Lloyd Alexander's Chronicles of Prydain comes to mind:
The Book of Three (1964)
The Black Cauldron (1965)
The Castle of Llyr (1966)
Taran Wanderer (1967)
The High King (1968)
The Foundling and Other Tales from Prydain (1970)
There's also Joy Chant's House of Kendreth set:
Red Moon and Black Mountain (1970)
The Gray Mane of Morning (1977)
When Voiha Wakes (1983)
though opinions on those differ. And, as you didn't say what (to you) comprises a series, I'll throw in Alan Garner's Alderley books:
The Weirdstone of Brisingamen (1960)
The Moon of Gomrath (1963)
Those are the ones that come to mind right off, though I'm sure others will be able to give considerably more recommendations, as this area is definitely not my strong suit.....
However, this may help:
http://www.chronicles-network.com/fo...k-readers.html