I've been wrestling with the Language Poets for several years now. It's a pretty masochistic pursuit, but I believe if one is going to write poetry in the 21st century (speaking of masochistic pursuits) one must come to grips with them one way or another. For good and ill, they bent the perameters of the art in such a fundamental way that even vehemently non-experimental poets must take their ideas into account.
Briefly put, the Language Movement was a reaction against a poetics developed by the Moderns that by the 1950s and 60s had become rote and self-indulgent. The Language poets valorized everyday speech patterns and idioms over self-consciously "poetic" language. Influenced by structuralist and post-structuralist theory, they saw the poem as a construction of language rather than a machine of meaning. In the Language Poets view, the degree to which a poet's artistic intent is realized in a poem is no longer the measure of its success: meaning is what the reader makes of a poem, not what the poet builds into it. In fact, given post-structuralist gaps between signifier and signified, as well as author and reader, (self)expression is problematic if not impossible. If words are constructs divorced from objective realities, then poetry becomes an abstract art. The play of language at the phrasal and word levels is foregrounded. It is aggressively non-linear. If the Moderns implemented a poetics that mirrored social and psychological fragmentation, the Language Poets argued a complete atomization.
Okay folks, it's time to sit bolt upright in that straight-backed chair, button that top button, and get set for some difficult poetry.*
Versed  / Rae Armantrout
From the Other Side of the Century: A New American Poetry, 1960-1990  / edited by Douglas Messerli
The Language of Inquiry  [electronic resource] / Lyn Hejinian (Hejinian's poetics)
Silliman's Blog  / Ron Silliman (Good interviews and opinions by a leading (if not the leading) theorist of the Language Movement)
Poems for the Millennium  / edited by Jerome Rothenberg (this two volume monster could choke an ox, but it is as complete a taxonomy of post-modern poetry as we are likely to find).
Gertrude Stein  was and is a major influence on the movement. Though she is not known primarily as a poet, many of her short works, operas and plays work as poetry.
Writings, 1903-1932 : Q.E.D., Three Lives, Portraits and Other Short Works, The Autobiography of Alice B. Toklas / Gertrude Stein.
Writings, 1932-1946 : Stanzas in Meditation, Lectures in America, The Geographical History of America, Ida, Brewsie and Willie, Other works / Gertrude Stein.
Last Operas and Plays  / Gertrude Stein
For more coherent explanation of the Language Movement see:
A Brief Guide to Language Poetry 
L=A=N=G=U=A=G=E P=O=E=T=R=Y  at PoetryPreviews.com
*This line is a blatant and shameful rip-off of a piece by Laurie Anderson .