It's been many, many years since I first read this series. It was one of my favorites as a child, so I just recently bought the boxed set to work my way through it again.Over Sea, Under Stone
is, if I recall correctly, not really part of the main series, being more of an introduction to the war between Light and Dark, with few of the characters appearing in the later books, except of course for Merriman Lyon. I remember even as a kid thinking that this was the least interesting book in the series, and rereading it as an adult, it was decent light entertainment, but very much a fairly typical children's adventure. Three pre-adolescent children, Simon, Jane, and Barney, visiting a Cornish seaside town on vacation, are caught up in an adventure when they discover a mysterious map, which their Great-Uncle Merry tells them is a clue to a priceless treasure that Must Not Fall Into the Wrong Hands. The justification for the wise and ancient Dumbledore figure letting three kids get themselves involved with a battle against the forces of darkness that they barely understand is a bit thin, but it's typical for this kind of book.
These early depictions of Merriman (who is the only character of major importance in the later books) and relatively banal servants of the Dark are a gentle easing into the series for younger readers. There isn't much in the way of magic in this first book: supernatural elements are only hinted at. The kids are plucky and clever about gathering clues and finding the MacGuffin, but mostly they benefit from nick-of-time rescues and the reader's knowledge that even minions of evil won't really
hurt children in a children's book. But for all that, it's a fairly intelligent book and draws on old Arthuriana and ancient history, so definitely one I can recommend to kids who are willing to read a book that's rather more thoughtful than exciting.
I'm going to plunge ahead into the other books, but with the hope that they pay off, and that this first book isn't an indicator of the entire series being simply not as interesting and fun as it was when I was twelve.