I just finished reading David Hughes' updated edition of "The Greatest Sci-Fi Movies Never Made", which documents the historis of film projects that either never got made, or eventually got made in very different form than what they started as (the umpteen attempts to make "Dune", for example).
Upon finishing this book I found myself surprised by how glad I was that many of these projects didn't get made. While it's difficult to judge the potential merits of a script on the cursory summaries herein, many of the projects described—in the words of their own creators—come across as terrible and wrong and often rote repeats of expected norms (action beats every ten minutes, etc.).
If anything, the book is most illuminating in regards to how the system works with regards to how projects are pitched, developed, revised, put into turnaround—wash, rinse, spin, repeat—and how changes in studio management can turn a "go" into a "no" right before the production is to start, and how directors take on a project not because they want to make the script in question, but because they want to make a mainstream movie or a big paycheck or just to work with certain people, and immediately demand that a greenlighted script get rewritten to their tastes.
The book's got a handful of factual errors as well, which isn't surprising given the number of subjects and contradictory sources. Finally, a couple of the chapters felt really thin, where the author had an interesting film project to talk about, but clearly didn't have enough information tofill out a chapter. This is most noticeable in the chapter about Star Trek films that never were, as most of the chapterdescribes how the films that were made got made, and gives short shrift to the more interesting topics of those that didn't: notably Philip Kaufman's 1977 "Planet of the Titans" (which got as far as pre-production) and Harve Bennett's "Starfleet Academy".
Overall an interesting read, but one that makes you wonder why anyone wants to deal with the Hollywood system.