Artist signatures are an important research tool if you are trying to identify someone based on a piece of art. Below is a list of reference works on the monograms and signatures of artists if all you have is a signature or marking for the artist.
This image shows the evolution of James McNeil Whistler's signature. Whistler was an American painter, and his signature became so popular that collectors would bring pieces they had already purchased back to him so he could do the signature again.