Over Sea, Under Stone
The first one : Meet the Drews. Simon, Jane, and Barney. Plus their sort-of Great Uncle Merriman (Gumerry). On holiday, discover weird people and weird maps that lead to a treasure. As you do.
Works as a standalone adventure story, gang of kids plus mentor seeking a treasure and trying to beat the rather unpleasant people to it. Add in an interesting setting, the Arthurian element, and you've got a rollicking good read.
The Dark is Rising
The second one : Takes a few of the elements from OSUS and builds on the mythology and background of Merriman's lot, aka the Light, and the unpleasant lot, aka the Dark. All of this is seen through the last of Merriman's lot's eyes as he comes of age. Say hello to Will Stanton : He's the youngest of a very large family, turning eleven, and finding out that he's the last of a race of immortals who have magic powers. Oh, and he's also got to fulfil a prophecy by gathering a bunch of signs to aid them in their fight.
Not really an adventure book - a lot of the setting is very normal and mundane - family christmas, growing up a bit - but it's all about changing as a person and figuring out this new world of power and what lies beneath the surface of what's going on. There's even some repercussions of your actions stuff. Plus bucketloads of fascinating mythology. Spooky, and utterly brilliant.
Greenwitch
Numero trois : The Drews are back in Cornwall for another holiday, hoping to see Gumerry and the town they found the grail in. Only there's this new kid Will Stanton who seems to know Gumerry and knows way too much. And the Dark are back and after something they lost last time. Throw in a town festival/ritual that near-defines 'weird'.
This one's good for seeing the Drews who *thought* they had a good lead into the world of the weird finding out that they only know the barest surface and it's all a lot deeper and weirder than they thought. Plus you see what Will looks like to the casual observer, which is far different from how he appears in his own head. It builds on the mythology, the town's connection with it, other forces at play aside from the Dark-Light clash in the world and delves more into the Drews and how they've grown up a bit in the year since we saw them in OSUS.
The Grey King
Four : Will's been really sick, and gets shipped off to his aunt's place in North Wales to recover and get his strength back. Only he's met this really odd lad on a mountain, Bran Davies, who's got a whole bit of one of those prophecies about *him*. And did we mention the Dark are abroad again and Merriman's nowhere to be seen? (this one won an award for being brilliant, btw)
Expansion of the legend, especially the Arthurian elements mentioned in passing in previous books (we'll explain later), and delves into a bunch of Welsh mythology as well as the Cornish and English previously touched on. Bran's family life and his position as a loner and outcast, family and community ties, death, estrangement, manipulation through arrogance and what happens when you've got forces battling over your community - how it actually can really screw things over.
Silver on the Tree
Five : Drews, on holiday *again* - this time in Wales, and who's this arrogant sod Bran Davies that Will knows? And... er... did we mention that this time the battle between Light and Dark is the big one? Oh, and at one point Will and Bran get dragged off to a land that was last seen being drowned when the flood walls broke while the Drews have to keep everything together back home. Not so much a race against time as trying to keep hold of your sanity by the skin of your teeth.
Yet more Welsh mythology incorporated! (oh, hush, it makes for a good read) The Drews have pretty much accepted that they're never going to have the full story on anything since they only holiday in this mad weirdness that Will and Bran and Merriman live in constantly, and are trying to keep everything together on this mad rollercoaster, and trying to help while they get buffeted by the Dark who figure they can get to the Light lot through them. We get an added dose of the madness that is the Lost Land (seriously, exceedingly freaky) while building on Bran coming into his own when he knows more than Will - because Will's now settled into his role as supporter and backbone for Bran. And then we get the repercussions of what happens at the end of the adventure.