Detour (1945)
Second Unit
A prominent example of what was viable back in the day, slightly before such books became superfluous/nonviable, were the X-Files "official guide" books from the mid-late 1990s.
The Official Guide to The X-Files Series by Brian Lowry
The Truth Is Out There (The Official Guide to The X-Files, #1), Trust No One (The Official Guide to The X-Files, #2), I Want To Believe (The Official Gu...www.goodreads.com
There were six "official guide" books which were episode guides compiled with behind-the-scenes type information for each season. (The first book cover seasons 1 and 2). The fact that these "official guide" books stopped publishing new volumes after season 7 of the show, is highly suggestive when this type of book became no longer viable for a big publisher (Harper Collins) after 2001.
And, being a huge X-FILES fan at the time (that passion being extinguished as Seasons 6-9 progressed) I picked up the first volume of this series.
I don't remember much about it, but I rated it 2/5, so didn't bother to continue picking them up.