The very first entry in Moldvay’s Basic’s Inspirational Source Material are three books by Llyod Alexander, the first three of his Chronicles of Pyrdain (interestingly the last two are not on the list, despite having been in print). The Book of Three and The Black Cauldron. The books, fitting true D&D tradition, are part of a coming of age series whose primary character is the newly appointed Assistant Pig Keeper Taran. At the beginning of The Book of Three Taran is a young man tired of his duties as a farm hand caring the oracular pig Hen Wen. When Hen Wen flees as the evil Horned King, a servant of the Dark Lord Arwan, is seeking her Taran takes off to find her. His adventures bring him various companions including the semi-bestial Gurgi and the Taran’s hero, Prince Gwydion. After capture by Queen Achren and the undead Cauldron Born they are imprisoned in Spiral Castle. Aided by her “niece”, the Princess Eilonwy, Taran escapes with yet another individual the king turned wandering bard Fflewddur Fflam (who Eilonwy mistook for Gwydion). The three head to warn the High King in Caer Dathyl with the Horned King in purpose. In the process of the resulting adventures Taran determines that caring for Hen Wen isn’t quite as bad as it seemed or adventure as fun as he thought.
The Black Cauldron begins with Arwan sets out to recover from the loss of the Horned King by creating even more undead Cauldron Born. Gwydion determines to end the process by destroying the Black Cauldron which is their source. His council gathers our companions from the prior book. An additional quester in the form of the arrogant noble Ellidyr sets off with them. Ellidyr eventually abandons the companions seeking glory in recovering the Cauldron himself. It is eventually destroyed but character is revealed and sacrifices made.
The books are aimed at young adults. In fact, The Black Cauldron is a Newbury Honor Book and the final book of the series, The High King, was a Newbury Award Winner. Disney would make an animated movie called The Black Cauldron based on the first two books.
The books draw heavily on Welsh mythology. Alexander was researching Wales for a different book and saved the research for this series instead. I would recommend these books, as well as Castle of Lyr mentioned by Moldvay and Taran Wanderer and The High King which he didn’t, for anyone interested in flavor for a Celtic campaign that eschew’s the New Agey Celts of modern fantasy novels. They are quick reads and engaging even for adults.
In terms of gamable material they are rich. Eilonwy’s bobble, the sword Taran wields at the end of the first book, and the Black Cauldron itself are interesting ideas for unique magic items. The Cauldron Born are interesting takes on zombies both in their capabilities and creation. Another set of Arwan’s servants, the Huntsman, who gain strength as their fellows die were a standard in my early 80s campaigns. I was apparently not alone as a version appeared in the Dragon’s Bestiary in Dragon issue 40. Fflewddur Fflam’s only rival for my model of a bard back then was the Harper Menolly from the Pern series.
I know they had a huge influence on my Silver Age gaming. I think a similar influence on modern games of OD&D could be a good thing.