<P>Most critics would agree that John Ashbery is one of 20th-century American poetry's finest voices. Perhaps his most admired book is <i>Self-Portrait in a Convex Mirror,</i> a culmination of themes, styles, and forms with which the poet experimented over the course of two decades. Now, the poet's devoted readers can trace his development through the first five books of his poetry, collected here in one handy volume. <i>The Mooring of Starting Out</i> represents Ashbery's work from 1956 through 1972, comprising <i>Some Trees,</i> his first book; <i>The Tennis Court Oath,</i> written while he was living in Paris.