I'm sure your brother and I would get on pretty well, Eari, though unfortunately I know very little french (though I can speak it pretty well phonetically!). I know some basic Turkish, since my wife is from Turkey, but that's about it besides English, alas. At any rate, I'm sure I could learn quite a bit from him. It must be nice living in France for a history enthusiast (I assume you live in France). There isn't much I wouldn't do for a trip to France - especially south France!
Celtic culture, you say. Indeed! I, too, love reading about it (as did Tolkien!). Maybe one of the few places that might even top south France for me might be Wales. As I mentioned, my historical piece I am writing begins there. Are you at all familiar with the legend of Madoc? Madoc ap Owain Gwynedd was an illigitimate son of king Owain Gwynedd in the second half of the twelfth century. He was renowned as a marriner and was reputed to have left the barbaric Wales with one of his brothers and several followers to sail off into the distant west across the Atlantic. There is evidence now to suggest that they succeeded in their oceanic crossing and at length landed in what is now Mobile, Alabama, along the gulf coast. At some point he (and later on his descendants) made their way up the river systems of the mid west and left behind them several stone fortifications - many of which still stand to this day, albeit mostly in ruins. Here in my own state of Indiana the remains of a very Welsh-looking fortification can easily be visited down along the Ohio River! I know this because my father and I have been there and stood upon what must have once been some sort of guard-post that was built there to watch for enemies along the river course. We even ate our sandwiches on it that day! It takes a bit of a mild hike through some underbrush in the woods to get to it but it wasn't a big deal. Anyway, the subject of Madoc is a passionate one fore me. If you want you can look up Madoc on Wikipedia, though I don't take kindly to whomever wrote the info on it, as they make light of the while thing and even suggest that the whole story is nothing more than an old fictitious tale.
I can relate with your difficulties on research. That's one of the drawbacks to writing historical fiction, I suppose. My main problem with the whole Madoc thing is my lack of knowledge of medieval sea-travel and the sea-craft they used. The crossing of the unknown ocean would be a big part of the tale, in my opinion. So I haven't figured out how to deal with it yet. That's why I am mulling over the prospect of trying to come up with a land-based tale (ie. Albigensian Crusade). we'll see. That is why I am so occupied with this North Kingdom tale - to see if I am capable of writing a full-length epic. Indeed, I plan on robbing from the better parts of this story in order to tweak it into my History piece.
Gavriel Kay. I knew I had heard the name before but wasn't quite sure of where I had heard of him. I checked Wiki, though, and so now I remember. I haven't read any of his books before, but perhaps a few of them would kindle my interest, especially The Song for Arbonne, which seemingly deals with the Albigensian Crusade.
As far as I am concerned I am fascinated with Celtic culture as well as the culture of the Muslims in their golden age (what with the doctors such as Avissene - not sure of his name in English) as well as the period when Salah Al-Din (saladin) took the Kingdom of Jerusalem which prompted the Third Cruisade.
I find the latter really interesting because of how Christian, Jews and Muslims were able to live under one rule without killing each other
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Yes, quite true. And Saladin was not the only such example of religious tolerance under a Muslim ruler. A pity the same thing cannot be said about Christianity? The Albigensian Crusade was certainly a disgusting and reprehensible example of the western religious mindset at the time. Self-righteous absolutism at its very blackest.

If I were to write about this topic I would find it difficult to resist the temptation of further blackening the reputation of Simon de Montfort and the papal legates that he associated with. Innocent III probably wouldn't fare much better, either!