From the Introduction:
The primary purpose of reading and writing haiku is sharing moments of our lives that have moved us, pieces of experience and perception that we offer or receive as gifts. At the deepest level, this is the one great purpose of all art, and especially of literature. The writer invites the reader to share in the experience written about, and in the experience of the shared language itself. (v)
It's a very basic definition, and the author is aware of it, of course, but it's nonetheless a good starting point.
The important words are "moments" and "pieces," indicative of the brevity of the haiku. A flashing insight into the reality and concreteness of an event/vision/experience cannot be overburdened with words.
Another important word is "gift," and a haiku is, like the tea ceremony, offered in the spirit of sharing a precious "something" in a fleeting world.
More excerpts soon as I go through the book!