Oberle, D., Guarino, N., & Staab, S. (2009) What is an ontology?. In: “Handbook on Ontologies”. Springer, 2nd edition, 2009.

Uschold, Mike & Gruninger, M. (1996). Ontologies: Principles, Methods and Applications. Knowledge Engineering Review, 11(2).

Roberto Navigli and Paola Velardi (2004). Learning Domain Ontologies from Document Warehouses and Dedicated Web Sites Computational Linguistics 2004 30:2, 151-179. (The authors present a method and a tool, OntoLearn.)