The essential book for anyone working with historical documents: clear, concise, and well organized, with advice for just about every situation. With the exploration of practices are reprinted examples showing the results. Since it was published in 1997, there isn't specific information on digital documents, but the principles are here and can be adapted.
(A confession: I adore this reference book, which I've read cover-to-cover. It's a pleasure to see information presented so clearly; and while the nitty-gritty of presenting historical documents isn't as enthralling to everyone as it is to me, it's delightful to see how editors of major projects dealt with the quirks and eccentricities of their material.)
Regardless what level of historical documentary editing experience, this text is THE manual that covers all aspects of documentary editing. I appreciate the actual discussion of a lesson immediately followed by excerpts delineating exactly how editors treated a text. The writing is clear and concise, all from front matter to back matter and documentary editing aspects in between, this text covers it.