These book buyers respond to ads that sell manuals, handbooks, and other reference materials on a wide range of engineering topics. The ads come in different forms: in print (as in engineering and other technical journals) by fax, and by email. The order value ranges from USD 35 for a single book, up to as much as USD 2,500 for a set of several volumes. The titles that these people buy are cumulatively tracked to determine which one of the following engineering area is the book buyer’s main interest: aeronautical, civil, chemical, computer, electrical, electronics, genetic, marine, mechanical, metallurgical, mining, nuclear, sanitary, telecommunications, etc. Source: mail order houses.