The Jepson Manual: Vascular Plants of California, Second Edition, provides taxonomic treatments and dichotomous keys for members of the Hydrangeaceae that occur in California, including Whipplea.
Preston, Robert E. and Charles F. Quibell. 2012. Hydrangeaceae inThe Jepson Manual: Vascular Plants of California, Second Edition.B. G. Baldwin, D. H. Goldman, D. J. Keil, R. Patterson, T. J. Rosatti, and D. H. Wilken, Eds. University of California Press. Berkeley, CA.
Keys and treatments are provided online through the Jepson eFlora:
http://ucjeps.berkeley.edu/cgi-bin/get_IJM.pl?key=155(Link to identification key and taxonomic treatments for the Hydrangeaceae of California, including Whipplea.)
http://ucjeps.berkeley.edu/cgi-bin/get_IJM.pl?tid=48534(Link to the taxonomic description for Whipplea modesta, including maps of its distribution and specimen collection records).