I am not sure what you mean by "the grammar point", however the meaning of this sentence is that the greatest Frederick William Frohawk's contribution to the field of natural history during the Victorian period was his "butterfly illustrator" (the book that he apparently wrote). My guess is that the reference to the book is not completely correct. According to Wikipedia: Frohawk was the author of Natural History of British Butterflies(1914), The Complete Book of British Butterflies (1934) andVarieties of British Butterflies (1938).