Inside the Wicker Man, How Not to Make a Classic by Allan Brown  Probably the best single resource about the movie.
Robin Hardy created a graphic novel using the complete set of storyboard images for the film. We hope it will be published it in the future.
(A page from the sequel - very low res. Page 341 in Brown's e-book)
Studying The Wicker Man, Instructor's Edition by Andy Murray and Lorraine Rolston  A teacher's guide for a film studies course on the movie. Has outdated and incorrect info. Published in 2002.
The Quest for the Wicker Man, Historical, Folklore and Pagan Perspectivesby Benjamin Franks, Stephen Harper, Jonathan Murray and Lesley Stevenson  Another academic look. Read Chapter 2 here. (A review.) (See the page on the conference it came from.)
Constructing ‘The Wicker Man’: Film and Cultural Studies Perspectives, Edited by Jonathan Murray, Lesley Stevenson, Stephen Harper and Benjamin Franks. (See the page on the conference it came from.)
The Wicker Man: Conversations with Robin Hardy, Anthony Shaffer & Edward Woodward by Stephen Applebaum (available as a Kindle download)
Religion and Film: An Introduction, Hardcover – 27 Oct 2006 by Melanie Jane Wright, ‘The Wicker Man’, Religion and film: an introduction (London: I.B.Tauris, 2007)[Google Books preview, some pages omitted]The Wicker Man at Screenonline (BFI)
The Golden Bough by Sir James George Frazer. touted as a source of research by Robin Hardy and Anthony Shaffer.
The Folklore Fallacy. A Folkloristic/Filmic Perspective on The Wicker Man by Michael Koven (a "folkloric amusement park"... - From The Quest For the Wicker Man book.
Gail Ashurst (author of the Nuada newsletters) academic paper: In the footsteps of 'The Wicker Man': personal mythopoesis and the processes of cult film fandom
A Burning Passion Thats Close To Worship from the TES (Service not available, 9/27/15)
A review by Paul Binnion, University of Nottingham, UK (Search for it. It's near the end of the document.)
Catterall, Ali & Wells, Simon. "'The True Nature of Sacrifice' from Your Face Here: British Cult Movies Since the 'Sixties". Fourth Estate, 2002, (BK), 0007145543
Peary, Danny. "the Sleepers, the Weird and the Wonderful". In: "Cult movies 2: 50 More of the Classics", Dell New York, 1983, (BK), 0440516323, 1989, (BK), 038529753X, 1984, (BK), 0091544416
Withnail and Us: Cult Films and Film Cults in British Cinema by Justin Smith has a chapter on the Wicker Man. (Google Books version with pages missing)
Smith, Justin (2008) "The Wicker Man" digest: a web ethnography of a cult fan community. In: Chapman, J., Glancy, H. and Harper, Sue, eds. The new film history:. Palgrave Macmillan, pp. 229-244. ISBN 9780230594487 (or go to Academia.com, register and go to Justin Smith, U of Portsmouth for the download. It's free.)
Calculated Risks: Film Finances and British Independents in the 1970s - Justin Smith. From the Journal, Historical Journal of Film, Radio and Television, Volume 34, 2014 - Issue 1: The Film Finances Archive or Academia.edu.