I've been receiving a few late-running Kickstarter campaigns over the past few months and I was struck at the notion that some of them were quite brilliant, but since they didn't hit the shelves until after Second Edition was released (or announced) that they may see little use at most tables and may quite possibly remain undiscovered by the greater gaming community at large.
Two that spring to mind are the brilliant city-scape campaign book, City of 7 Seraphs, which details an extra-planar city that works as a campaign capstone for planar adventures.
The second would be the still running late Kickstarter book, Legendary Planet, from Legendary Games. Aside from Iron Gods, it's the only sword and planet type of adventure created for the Pathfinder system. It has been released in bits and pieces, but with the final book only recently being completed, we are still several months away from the compilation book being made available.
What are the great books you've come across that were released so late in the cycle that they may never get a fair shake of the dice at most tables?
Well because of a new rule for our gaming group that we can't use anything made after the current AP we are playing makes a lot of later things a lot less useful.
That sucks. I run a pretty open table and allow a lot of third party content like Psiomics, Path of War and the Spheres of Power series.
I don't blame GMs for limiting their tables because players will try to take advantage of things and if they are argumentative as well, this makes for a poor gaming atmosphere.
There is definitely that fine Player Companion Chronicle of Legends. While the capstones could definitely see play in any campaign that reaches 20th level, I doubt that many people will be in a position to do anything with the exemplar traits since you basically have to build your character around them from 1st level.