I picked up Celtika because I really enjoyed one of the author’s other works, Mythago Wood, and the library didn’t have any further entries in that cycle.  My nose wrinkled a bit at the series being called the Merlin Codex, because at this point, hasn’t everyone done a Merlin deconstruction? 
But what caught my attention is this is not only a Merlin slash Arthur story, it’s a crossover with Jason and the argonauts! The traditional story of Jason ends with him friendless and broken, bludgeoned to death by the falling prow of Argo. Only in Celtika, there’s more. Argo slips its moorings and sails off into the night with Jason’s limp form. No one sees it for seven hundred years, during which the ship hides at the bottom of a lake in the far north, a lake where vision-seekers gather because of the screams which emanate from it — the screams of Jason, still alive in a hibernation cum nightmare.
Along strolls Merlin, who sailed on Argo under the name of Antiokus. He’s been wandering the world for a long, long time already, hoarding his age for unclear reasons. He gets the sense this screaming lake is something he ought to look into and further intuits it’s his old friend Jason. Merlin convinces Argo to resurface. On recovering from a seven hundred year nightmare, Jason resolves to gather a crew and sail off in Argo to find his two long-lost sons, who Merlin happens to know have also survived to the then-present day, thanks to the craft of their mother, Medea, who wanted to take them out of Jason’s reach completely.
The story goes on from there blending Celtic and Greek stories. One of Argo’s crew is a Celtic chieftain named Urtha with whom Merlin finds a bond growing. He also becomes reluctantly entangled with a northern girl, Niiv, favored of Argo’s new, unwilling patron goddess and infused with an unsettling amount of magical oomph. The search for Jason’s sons takes the ship’s crew from the far north to Alba, then east across Gaul and right across down through the Hot Gates into Greekland.
It’s a neat read, but dense in an odd way. The protagonist is clearly Merlin, but I was mostly interested in the people around him: Urtha and his complex web of responsibilities, Jason’s utter inability to recognize when he’s being a reckless ass and the others aboard Argo. I’m glad I stuck with it and will certainly pick up The Iron Grail in due course — unless I find Lavondyss, the next book in the Mythago cycle, first.
 One of the risks one takes with public libraries. They’re great for saving cash and finding books you might not normally have run across, but it’s so easy to become invested in something and then not be able to follow up on it.
 I say that having only read one or two myself, with Mad Merlin and Arthur, King of Time and Space being the titles I can call to mind.