Although many of these little poems are three lines, they are not haiku. Instead, Harrison and Kooser mostly wrote back and forth to each other in epigrams and aphorisms.
Personally, I prefer the juxtaposition in haiku-like forms over the sweeping "truths" of aphorisms, or even the wit of an epigram. I think it's because the duality of the images when bounced off each other make the poem expand so far beyond the page.
Yet, there are some gems here, and some do have a haiku-like flavor. One of my favorites:
the bear's heart
in her footprints.
This was written by either Harrison or Kooser. None of the poems are attributed. There is a statement on the cover of the book, which was a response from one of the poets when asked who wrote what, "This book is an assertion in favor of poetry and against creditials."