Weidauer, L. Problemeder frühen Elektronprägung, Typos I. (Fribourg, 1975).
Probleme der frühen Elektronprägung (Problems in Early Electrum Coinage) is one of the standard references, frequently used in coin attributions, for what are arguably the world's first coins. The text is in German but a project to translate it into English is discussed here: https://www.forumancientcoins.com/board/index.php?topic=57442.0