When the International Field Guides database reached 5,000 records in summer 2007, Bird field guides were the most common at 17%, although birds make up less than 1% of all known species of organisms! By contrast, 7% of field guides cover Insects, which make up about 60% of all species but are more difficult to identify in the field. The most popular plant guides were Wildflower guides (12%) followed by general Plant guides (10%). North American field guides made up nearly half of the database, followed by 19% European guides. While the classification percentages are probably fairly accurate, the regional percentages almost certainly under-represent some regions of the world, especially those that use non-Roman alphabets and/or are not in areas actively collected by North American science libraries.
Schmidt, Diane. 2006. Field Guides in Academe: A Citation Study. Journal of Academic Librarianship 32(4):274-285.