Simon, Jane, and Barnabas Drew come to Cornwall on a summer holiday with their parents and their great-uncle Merry, a noted historian. While exploring their rented house on a rainy day, they find a hidden door leading to a neglected attic, and under a floorboard in the attic Barney comes across a mysterious manuscript, written partly in faded and obscure Latin, and partly in a language none of them can read. They don't want to tell their parents about it, as that would probably mean it would be taken away from them on the ground that it belongs to the owner of the house and they shouldn't disturb it; Barney in particular, having recognized the names of King Arthur and King Mark amongst the Latin, is adamant not only that the manuscript represents a quest but that the quest is theirs and no one else's. Fortunately Great-Uncle Merry is in a different class of grown-up, and not only supports their right to follow the map, but translates the instructions for them and watches over them when other parties start to show too much interest. Then Merry is decoyed away, and the children are left to follow the clues and avoid the competition on their own.
Of course the one I really wanted to read over the Christmas holiday was the second in the series; the summer vacation described in the first book was thoroughly unseasonable, and it was hard to feel too bad about Barney's sunburn. I seem to remember that this was another series where as a kid I read the second book first, and I don't think it matters much until you get to book three or four, but I'm not sorry to have started with the first in the sequence this time.
And there are children's books that don't condescend, that adults can also read again and again and get more out of them every time because you bring more to them. Over Sea, Under Stone and the rest of its series are like that.