As I recall George Kramer wrote a fine work on the telegraphs. An older work is entitled United States Telegraph Issues by Joseph and Stephen Rich. Somewhere in my library I think I have both, but I have not looked at them in many years.
Robert A Siegel Auctions sold a big collection of telegraph stamps about 5 years ago. The auction catalog is a good reference https://siegelauctions.com/sales.php?sale_no=1102(you probably want to download the .pdf file since it has information about the companies in section headings that are not contained in the on-line catalog's individual lot descriptions).