Books with illustrations may be identified using the Advanced search in Orbis, with Keyword Anywhere selected for the field.  Enter search terms such as conservation and (illus ill) or (bird birds aves) and (illus ill). Choose "Any of these" for the words in parentheses.

For sources on illustration or photographic technique or equipment, see A Researcher's Guide to Scientific and Medical Illustrations, 1990 by Mary Helen Briscoe. Springer Series in Contemporary Bioscience, Springer-Verlag, 1990 location  

Butterflies of the World by Adrian Hoskins 2018 with 350 color images of living butterflies location

Many illustrations can be found through google images or yahoo images also.

There are also reference books with pointers to illustrations in books. 

Index to Illustrations of the Natural World by John W. Thompson 1977 location 

Index to Illustrations of Animals and Plants by Beth Clewis1991 location 

Index to Illustrations of Living Things Outside North America: where to find pictures of flora and fauna by Lucile Thompson Munz, 1981 location