The following are some recommended sources for further information on Sam Peckinpah and his films. I will be adding to this list as I acquire further items.

Books:

Passion & Poetry: Sam Peckinpah in Pictures; by Mike Siegel (Berlin: Schwarzkopf & Schwarzkopf, 2003) A superb illustrated work, with some text in German.  Hundreds of images including poster and lobby cards, productions still, deleted scenes and personal photos.  See the "links" section for more information.

Sam Peckinpah's West: New Perspectives; edited by Leonard Engel (University of Utah Press, 2003)

The Films of Sam Peckinpah; by Neil Fulwood (London: B.T. Batsford, 2002) A good introductory book to Sam's films.

Sam Peckinpah's Feature Films; by Bernard F. Dukore (Chicago: University of Illinois Press, 1999) A solid analytical book on Peckinpah cinema.

Savage Cinema: Sam Peckinpah and the Rise of Ultraviolent Movies; by Stephen Prince (Austin: University of Texas Press, 1998)
Peckinpah's style and technique are examined in detail, with the most notable analyses being The Wild Bunch and Straw Dogs. Also discusses Peckinpah's impact on the development of violence in the cinema. A highly technical book; recommended especially for students of film.

PECKINPAH The Western Films A Reconsderation; by Paul Seydor (Chicago and Urbana: University of Illinois Press, 1997)
An outstanding analysis of Peckinpah's Western films. Thorough and exhaustive by a man who knew Peckinpah personally. Mr. Seydor is also an accomplished editor whose credits include Tin Cup, Cobb, and White Men Can't Jump. This is an expanded version of the book which was originally published in 1980. Highly Recommended.

"If They Move...Kill 'Em!" The Life and Times of Sam Peckinpah; by David Weddle (New York: Grove Press, 1994)
An excellent biography, culled largely from interviews with friends, relatives, and co-workers by the author. Get a copy of this book while it's still in print.

Doing it Right: The Best Criticisms on Sam Peckinpah's The Wild Bunch; edited by Michael Bliss (Carbondale and Edwardsville: Southern Illinois University Press, 1994)
A collection of essays and reviews on The Wild Bunch. Includes contributions from Robert Culp (Introduction), Paul Seydor, Paul Schrader, Michael Bliss, Stephen Farber, Jim Kitses, Cordell Strug, John L. Simons, Aljean Harmetz and Michael Sragow.

JUSTIFIED LIVES- Morality and Narrative in the Films of Sam Peckinpah; by Michael Bliss (Carbondale and Edwardsville: Southern Illinois University Press, 1993)
A film-by-film analysis of all of Peckinpah's works. Well worth seeking out.

Bloody Sam; by Marshall Fine (New York: Donald I. Fine, 1991)
This biography has been criticized for putting too much emphasis on Sam's partying and crazy lifestyle. Perhaps, but it still does contain good interviews along with some often humorous anecdotes.

Heroes' Twilight: The Films of Sam Peckinpah; by William Parrill (Hammond, Louisiana: Bay-Wulf Books, 1980)

Documentary Films:

The Wild Bunch: An Album in Montage
Editor Paul Seydor fashioned this outstanding documentary around some black and white footage shot during the making of The Wild Bunch. Originally shot without sound, the film features over-dubbed commentary by actors and crew members from the film. Scenes from the film are also intercut throughout. (This film was nominated for an Academy Award.)

Sam Peckinpah: Man of Iron
A great documentary consisting almost entirely of interviews with former Peckinpah associates. Included are James Coburn, Jason Robards (reading from Peckinpah's diaries), Kris Kristofferson, Ali McGraw, L.Q. Jones, R.G Armstrong, and many others. This airs occasionally on the A&E Network. An altered version (minus some film footage due to ownership rights), is included on the Criterion STRAW DOGS dvd.

Additional Books:

Peckinpah: A Portrait in Montage; by Garner Simmons (Texas Press, 1982)
A biography/film-by-film critique. Highly recommended.

Sam Peckinpah; by Doug McKinney (G.K. Hall & Co, 1979)
A film-by-film critique. One of the "Twayne's Theatrical Art's Sereies" which detailed various directors careers.

Crucified Heroes: The Films of Sam Peckinpah; by Terence Butler (Gordon Fraser Press, 1979)
An obscure British paperback containing an analysis of Peckinpah's films to that time.

Sam Peckinpah: Master of Violence; by Max Evans (Dakota Press, 1972)
Written by renowned Western author Max Evans. Consists primarily of anecdotes relating to his time spent on the set of The Ballad of Cable Hogue. This is an extremely rare book.

Horizons West; by Jim Kitzes (Indiana University Press, 1969)
A review of the western films of Anthony Mann, Budd Boetticher, and Sam Peckinpah. One of the most highly-touted books on Western films ever published.