To a great extent An Introduction to Poetry (Hubbell and Beaty) was used as a college text in the 1920s, 30s and 40s and is a composite handbook of prosody and anthology. It covers the subjects in a logical and methodical way by being straight forward and it included many poems from outside the canon. They use canon as a loose term for "the poems people usually include in anthologies". One of the first questions the handbook introduces is "What is poetry?" and quote several that are widely known:

The poem dealing with this subject that almost all authors mention is in Ars Poetica. by Archibald MacLeish which was published in 1926 and claims that poetry is to be enjoyed for the same reasons that one enjoys nature -- simply because it is.

Comprised mainly of images -- simple word pictures -- it is intended to be read and understood at many levels by everyone, but not necessarily as the poet initially meant. The art is in the image the poet has drawn. Though the poet cannot predict the readers specific reactions or understandings, it then becomes the reader's responsibility to look at poetry more than read it. A fun thing to try is to rewrite the poem, changing at least one noun per line, deleting words like the, a, an, when, next, then, etc. It helps the reader to form a personal connection. By composing an eight line poem, beginning with A poem should be equal to: / Not true, I used images to suggest my personal interpretation of MacLeish’s poem.

A poem should be equal to:
Not true

For obsidian
rocks would remain unmoved
as glassine reflections

For all the cardinals
boundless in their flight of song

A poem should not mean
But be.

Try it you might be surprised that you too can discover 'the art of poetry.'

Sources:

Blair, Bob One of the first questions Hubbell and Beaty tackle is "what is ... :
www.geocities.com/~bblair/001113f.htm