Lakoff, George, and Mark Turner.  More than Cool Reason: A Field Guide to Poetic Metaphor.  Chicago: University of Chicago Press, 1989.

Martin, Wallace.  "Metaphor."  The New Princeton Encyclopedia of Poetry and Poetics. Eds. Alex Preminger, Terry V. F. Brogan, and Frank J. Warnke. Princeton, NJ: Princeton UP, 1993.  760-66.

Metaphor

What is a metaphor?

Metaphor (classical rhetoric)

What is a metaphor? (linguistic)

Metaphor and Simile (by Tim Love; examples and discussion)

Basics of Metaphor and Simile

Kalliope: Focus on Metaphor (definitions, examples, exercises)

The Logic of Tropes (metaphor, simile, metonymy, synecdoche, etc.)

Jaron's World: The Meaning of Metaphor ("A new theory may illuminate the nature of meaning"; cognitive angle on metaphor)