Hathi Trust has multiple versions of the Illustrierte Flora von Mittel-Europa (Illustrated Flora of Central Europe) that Gustav Hegi edited between 1908 and 1931. There are editions published after his originals as well. I chose to peruse the volume made available by University of Michigan. The first 6 volumes (some of the volumes have multiple parts so there are more that 6 items in all) have colorful images – lots of plants on one page. The sample images from the 12 volumes I looked at – lots of color and variety on a winter day.
According to Wikipedia, the author, a Swiss botanist, wrote about a third of the content…and edited the whole. He obtained his PhD in 1905 and was had been a curator at the Botanic Garden of Munich from 1902-1908. The volumes were published in Munich. He died in 1932. The article contains very little information of his life outside of his publications although he had returned to Switzerland before he died at age 51.