For the true lover of entomological etymology. The Century Dictionary is a treasure-trove of 19th century science and technology, including lots of zoology and botany. The origin of the names of obscure insect genera can sometimes be found in this work. (I had been unable to find the origin of Bomolocha anywhere else.) A mammoth, leather-bound, 10-volume work, long out of print, but available in university and large public libraries. A great source of public-domain zoological art as well, it is profusely illustrated with very detailed engravings. This was the American answer to the British Oxford English Dictionary (OED). Supposedly the editors of the OED left the technical material to the American work because it was so superior to their efforts.

Update. Now available on-line, and they also sell a CD-ROM.