Reddit mentions of A Guide to Western Historical Scripts from Antiquity to 1600

Although uncial ultimately derives from Greek, the insular form (what you're calling Celtic, though isn't usually referred to as such by scholars) got to the British Isles through the Roman Empire. It wasn't until the 4th century that it really started getting to the form we recognize it today.

The ductus (i.e. the letter-forms) that the Celtic peoples Caesar encountered used was probably not uncial. It could have been, but that's not where I'd place my academic bet.

Addendum: A great place to learn more about all of this would be Bernhard Bischoff's Latin Paleography: Antiquity and the Middle Ages. Fair warning, that book is SUPER dense. If you want something a bit easier to get through, but not nearly as in-depth, you could look into Michelle Brown's A Guide to Western Historical Scripts. It is a really good learning handbook. Bischoff's is a nuts and bolts history. The guide is a practice tool.

I think I know what you mean. For broad edge, I'd suggest Michelle Brown's A Guide To Western Historical Manuscripts. I'm not sure what the answer is for pointed pen.

Brown's book is meant for paleographers (historians who read old manuscripts) not calligraphers, so it won't have anything like a ductus or comments on how to do the scripts inside. What it does have is a crap ton of scripts, laid out in high-quality full-page photos from the original manuscripts in a nice large format.

If you want instructions for how to do said scripts, I'd recommend The Historical Source Book for Scribes, by Brown (again) and Patricia Lovett, an accomplished calligrapher. This won't have quite the breadth you're after (it only has 14 scripts vs. 55 in Guide to Western historical manuscripts), but I think it's a valuable starting point to learning scripts from just a manuscript, as I talk about at length here (that comment also has some links to online libraries with extensive digitization projects; it takes some work to get what you want out of them, but once you do the selection and image quality is miles better than any book).

If you can't find an affordable copy of "historical source book" (the price seems to fluctuate wildly), Drogin's Medieval Calligraphy is not as good, but still a good starting point. And it's super cheap.

Also, I wouldn't recommend Harris' The Calligrapher's Bible. It's overdone in some areas and underdone in others, as I talk about here.

Sorry to link to my own replies so much, but I have a lot to say on this topic and I only have so much time to type :)