A week or two ago, Norman Spinrad made his unproduced script, He Walked Among Us, available as a Kindle ebook. I've bought it but haven't read it yet. So I won't review it now. Instead, I'll blather a bit on the subject of unproduced scripts.
One of the lost Star Trek books I really wish had been published is Allan Asherman's The Star Trek That Never Was. "Available at last! What you could never see on TV: adventures written for the original series and its planned sequel -- kept secret until now!" Scheduled for publication in October 1988 but pulled at the last minute, this book, according to its cover, "compiled" material by Spinrad and others. It may be a coincidence, but around the time the book should have been on shelves, a TNG episode based on an unproduced Star Trek Phase II script -- "The Child" -- was airing on TV. Years later we got a book by Gar and Judy Reeves-Stevens about the unproduced Phase II series, with some information on episodes that would have been included in Asherman's book, but Pocket never released anything like the Asherman book.
Back in 1985 Files Magazine, one of the incarnations of the Hal Schuster empire of poor quality unauthorized tie-in books, published two very thin magazine-sized trade paperbacks by John Peel under the titles The Star Trek That Almost Was and The Star Trek That Never Was. The latter had a handful of pages on Phase II, but the former described several unproduced Star Trek episodes from the 1960s.
The stories Peel summarized are "Rockabye Baby, Or Die," by George Clayton Johnson (who wrote "The Man Trap" and several Twilight Zone episodes), "The Joy Machine" by Theodore Sturgeon (who wrote "Shore Leave" and "Amok Time" and many classic science fiction stories and novels; this was turned into a novel by James Gunn); "The Lost Star" and "The Godhead" by John Meredyth Lucas ("The Changeling" and several other TOS episodes), "Shol" by Darlene Hartman, "Pandora's Box" by Daniel Louis Aubry, "The Aurorals" by Frank Paris, "Perchance to Dream" by J.M. Winston, and "He Walked Among Us" by Norman Spinrad ("The Doomsday Machine" and several classic SF stories and novels). If you ever saw the summaries of some of these episodes over at the Forgotten Trek website, they were reproduced from this book without attribution. They don't seem to be on the new version of the site.
In 1992, Cinemaker Press published Lost Voyages of Trek and The Next Generation, as by Bill Planer (actually Ed Gross and Mark Altman, according to the UK ediiton). Though Gross had worked for Schuster's content mill, his books with other publishers were often well researched and useful, and this one is no exception. (Gross did recycle some of his content from book to book, though.) The book starts with a five page summary of Spinrad's "He Walked Among Us," but it's the only original series TV script discussed. The rest of the book covers Phase II and unfilmed Next Generation episodes.
David Gerrold had a few stories that never made it past a pitch session, as described in his nonfiction book The Trouble With Tribbles; one eventually became a Star Trek novel, and another was adapted in one of Tokyopop's Star Trek manga paperbacks.
Meanwhile, former Pocket Books Star Trek editor John Ordover was quoted at Forbes.com, mentioning that he had copies of a few of these unproduced scripts in his Pocket office and that for all he knows they may still be there...