There are some good books out therre on the subject of artificial languages:

James Knowlson. 1975. Universal Language Schemes in England and France, 1600-1800.
Umberto Eco. 1995. The Search for the Perfect Language.
Marina Yaguello. 2001. Lunatic Lovers of Language: Imaginary Languages and Their Inventors.
Jean-Jacques Lecercle. 1990. The Violence of Language. (Not really about conlanging, but some coterminous fields of study.)

Ceci n'est pas un seing.