Thank you to Ari Mala and Bridget for feedback about painting in 3D - perspective, shadowing, whatever you want to call it.
Bridget offered a link to the blog Surface Fragments about how to paint trompe-l'oeil. It's excellent and helpful - so good, I'm posting it again, because I don't want anyone to miss it.
And: Ari Mala was commenting on working on acanthus painting (the twisty foliage so common in medieval and renaissance illumination). The same blog has four-post series about painting acanthus leaf!
Plus a post about how to draw an acanthus scroll (almost same curve as you see in a nautilus shell, or 'phi-spiral', apparently), which is hugely helpful.
Of course, after posting my previous 'I wanna learn' message I remembered there's a medieval how-to handbook for this - the Gottigen Modelbook.
It's online (the interface is slightly dated, but works in my browser), and it even compares the how-to book to the finished work in a copy of the Gutenberg Bible (this part, using Flash, doesn't work as smoothly for me).
Now back from the 20-year celebration, and what a splendid happy event it was. There's lots to say about it, but for here, I thank Mistress Bridget for organising the scriptorium - an airy room with good light and enough space to spread out.
The instruction I got in cutting quills from Mistress Caitlin will change how I handle them, and I'm itching to try it out. I've read many instructions about quill cutting - there's a short description in many books - but there's no comparison to getting personal guidance from a regular quill user...and using a really, really sharp knife.
Assisting in two courts, I saw such an array of beautiful work that it's hard to put into words - we have an embarrassment of scribal riches in this kingdom and should take pride in it.
ETA: still more links courtesy of Surface Fragments: a whole book, on archive.org, about drawing acanthus leaves, from a 19th c designer, James Page. Very much in the Victorian style, but beautiful drawings and etchings to supplement the wordy instructions.
Guide for drawing the acanthus, and every description of ornamental foliage; (1886)
This is a great fat file, no matter how you download it. But I recommend the drawing instructions (vs carving, engraving, mouldings, etc)