I am trying to find a book/website regarding 20th century Goldsmiths and their Makers Marks. Does such a thing exist? Don't Goldsmiths have to register their marks somewhere? Thanks!

I have not found any that are thorough. This has some The Goldsmiths' Company - Current Hallmarks (http://www.thegoldsmiths.co.uk/assayoffice/currenthallmarks.htm)
I have found mostly ones for silver and Native American jewelry.

You could try Amazon.com for books