Emacs has “info mode” documentation on “Picture Mode“.
“CUA Bindings” (cf. “info mode” entry by this name) come with enhanced rectangle support.
The O’Reilly book Learning GNU Emacs has a chapter (7) on Simple Text Edition and Specialized Editing with sections Rectangle Editing (subsection on CUA Rectangle Editing) and Making Simple Drawings (on Picture Mode and Artist Mode). Occasionally I actually use Picture Mode for drawing tables.
You will find info mode documentation on table editing in section Text > Text Base Tables. I also like that very much.