These above are not my books. I do own three out of these five, with these covers, but I also have a single volume omnibus edition of the entire series, which I couldn't find an image of online. That omnibus was published in 1984, when I was eighteen, and you might have thought I would be growing out of middle grade fantasy, but my edition is soft and battered and very well-read, so I mustn't have been too old for it after all.
I re-read The Dark is Rising (volume two, as well as the name of the whole series) a few years ago, as part of a Twitter (RIP) Christmas Eve collective read-through, an enriching and delightful experience. But I haven't read the other titles in the series for years.
As a young teen, I absolutely adored these books, especially the mixture of the everyday and the high stakes magic that evolved once Will Stanton appeared, an eleven year old boy who is also the last of the powerful Old Ones. I loved the simple but potent symbolism of the Six Signs that Will is charged to collect, and the inchoate sea-power of the Greenwitch and its connection to Jane, the only real female protagonist. I loved the enigmatic figure of Merriman Lyon (Merlin), Will's mentor, and the solemn, serious battle between the Light and the Dark.
The last two books of the series, The Grey King and Silver on the Tree, always appealed to me the least, especially the final, climactic volume, where Will and Bran (King Arthur's heir) enter a strange magical city and perform various symbolic ordeals -- this was the point where, for me, the story lost its grounding in reality, and even though the children are left to carry on the fight against the forces of the Dark in our own world, the nature of that battle is left pretty vague (there's only one stand taken against racism, which I did appreciate, but it's mostly Will's brother who fights that battle, so any role the children might take is left largely undefined).
It did feel horribly apt to be reading The Dark is Rising while Trump was being re-elected in the US. It's pretty clear what the battles for us are going to be.