There's an excerpt on his wikipedia page http://en.wikipedia.org/wiki/Martin_Luther_King,_Jr. then search the text for "anonymous", and an image of it at http://www.oilempire.us/cointelpro.html
TIME magazine also mentioned the letter in 1975 at http://www.time.com/time/magazine/article/0,9171,913763,00.h...
Title: "Judgment Days: Lyndon Baines Johnson, Martin Luther King, Jr., and the Laws that Changed America"
Author: Kotz, Nick
Publisher: Houghton Mifflin (2005). p. 247.
Access it here: http://books.google.com/?id=qo9r4_rnrasC&pg=PA247
[Looking at this URL again, is this an affiliate URL?]
There's an excerpt on his wikipedia page http://en.wikipedia.org/wiki/Martin_Luther_King,_Jr. then search the text for "anonymous", and an image of it at http://www.oilempire.us/cointelpro.html
TIME magazine also mentioned the letter in 1975 at http://www.time.com/time/magazine/article/0,9171,913763,00.h...