Modern popular/ mass art has rarely respected boundaries among disciplines and media. As soon as a music artist is commercially successful, film projects arise, as with The Beatles. Individual performers are even more likely to launch careers as actors, as with Frank Sinatra. Movies have their "tie-in" projects: toys, magazines, posters, television programs; Star Wars provided the template for this kind of maneuver, though plenty of films had been subject to such treatment prior to the late-1970's onset of the summer "blockbuster" era. This diversifying approach to an art project need not be entirely the result of commercial pressure. The television series Twin Peaks had numerous side projects, of which only one--a fake tour guide to the fictional city that is the show's namesake--seems to lack serious artistic purpose. Thus, the project Twin Peaks--not just a T V show--consists at least of:
the Julee Cruise album Floating Into the Night (1989) that features most, but not all, of the songs she sings as a character in the series;
all thirty episodes of the television series (1990-1991);
"Diane...": The Twin Peaks Tapes of Agent Cooper (1990) audio book [unfortunately only released as an audio cassette, now quite rare; some of the material comes straight from episodes, the rest is original];
Soundtrack From Twin Peaks: Music Composed by Angelo Badalamenti (1990);
The Secret Diary of Laura Palmer (1990) by Jennifer Lynch;
My Life, My Tapes: The Autobiography of F.B.I. Special Agent Dale Cooper (1991) by Scott Frost;
Twin Peaks: An Access Guide to the Town (1991); and
Twin Peaks: Fire Walk With Me (1992).
Furthermore, any archival collection of the series would preferably have the scripts of each episode and Fire Walk With Me; these plays, if you will, have an independent identity, however limited. Other tie-in products might have appeared as well.
[Spoiler Alert] As Greil Marcus has discussed in a fine essay on Twin Peaks in the otherwise-dreadful (by far his worst writing) The Shape of Things to Come: Prophecy and the American Voice, the film Fire Walk With Me is quite distinct from the series in its presentation of Leland Palmer's guilt in raping and murdering his daughter, as compared to the spirit Bob that possessed him. The film lacks the series's humor, almost entirely; even the early portion of the film, which features Dale Cooper, has only a few comedic aspects. So does Jennifer Lynch's novel. These two works stand apart from the rest of the project in this regard (not to mention certain, more-obvious differences between the film and the series, such as a different actress playing Donna Hayward and changes in the sets and filming locations).