C&I 101 Sources
David Harris' The Art of Calligraphy is my most highly recommended source for a beginning calligrapher. A print copy is invaluable, but it is available for free online. David Harris has another book that is reportedly comparable, The Calligrapher's Bible, but I have yet to read it myself.
Marc Drogin's Medieval Calligraphy is another oft cited source for beginning calligraphers. For me, it is less intuitive than David Harris' works, but still very valuable.
Insights Into Left-handed Calligraphy by Betsy Rivers-Kennedy. I haven't had much chance to assess this source, but it is one of the few sources I've found for lefties.
General Scribal Sources
The Craftsman's Handbook, Il Libro Dell' Arte, by Cennino d'Andrea Cennini. So often referenced, many scribes simply call it Cennini. This contains all sorts of instructions for supplies, especially in regards to paint.
On Divers Arts, attributed to Theophilus, whose true identity is unknown. Appropriately named, it describes pigments and instructions for their uses in MANY arts - as well as things like instructions on making a kiln for glass firing.
The Materials of Medieval Painting by Daniel V. Thompson is not a period treatise, but is a well-researched and easy to follow source. It doesn't just cover scribal painting, but also icons and furniture painting, among other things.
Understanding Illuminated Manuscripts by Michelle Brown is essentially a scribal dictionary. I haven't referred to it much, but it is useful to have on hand.
The Gottingen Model Book is facsimile of a 15th century illuminator's manual. It's the star of my collection. It has step-by-step instructions for common designs.