I used to confuse Walt Whitman with Wally Whyton, and was always vaguely puzzled as to why a performer of comic songs on 1970s British children's TV seemed to have some kind of iconic status in American literature. It may also be telling that none of Whitman's poetry (in fact, I think no American poetry at all) made it into the curriculum of my Belfast grammar school's English department. But I seem to be in Whitman's historical zone at the moment – this is the fourth book I have read since the start of June about nineteenth-century America – so I shall confuse him with Whyton no more.

Leaves of Grass is one of those seminal (I choose the word for reason that will become clear) works of literature that may be as important for its influence as for its actual content. I do not consider myself especially well-read in great poetry, but even I can tell that Yeats was drawing a lot from Whitman's well, both in terms of style (the free verse following thoughts through variations of meter) and substance (creating a national story steeped in history), and that the more experimental prose writers of the twentieth century, including Burroughs and Delany, owe a lot to Whitman's stream of consciousness approach and also to his frank sexuality. Indeed, the fact that a mid-nineteenth-century writer could adopt such a clearly bisexual perspective was the biggest surprise of the book for me.

Whitman's vision of America is pretty coherent, of manly white men (and their women occasionally) creating a new society on a new continent which will be a beacon of civilisation for the rest of the world to admire and emulate. The continent's previous inhabitants are preserved in place names – New York is always Mannahatta, Long Island is Paumanok. It is also striking that Canada (or Kanada) is part of the geographical vision. It's a fascinating example of a political programme expressed in verse.

It's therefore a shame that such a lot of it isn't actually very good. The early piece, "Song of Myself", which is the core of the original mercifully short 1855 Leaves of Grass, and the elegy to Lincoln, "When Lilacs Last in the Dooryard Bloom'd", were the two pieces that actually grabbed me. Otherwise, I found it frustrating that in many of the poems, even the shorter ones, a good opening concept gets lost in lists and in listless repetitions, with the occasional good line jumping out at you. Whitman is very easy to parody, but sometimes the original is so absurd that there is no need. As Rossini (or was it Shaw) supposedly said about Wagner's operas, there are some beautiful moments but some terrible half-hours.

So I'm glad I finally got through this, but would recommend that others interested in getting to know Whitman look for highlights, preferably in an anthology including other writers.