First, Welcome to AsciFi, I Brian. :wave:
I'll forego the usual recommendation to explore and post. You seem to have found most of the better places already!
Had to think about Terry Brooks' letter somewhat to understand what he is getting at and if he is being snide at the expense of other writers. It does seem quite dismissive of a certain style of writing
As a fantasy writer he must surely build a world for his story to work at all! And as so many of his works are serial in nature he would appear to be creating a lot of those bricks he complained about and is just as surely two-faced.
I thought he might be having a pop at the likes of Tolkien, who is probably a close competitor even if he is dead. Middle Earth shows the sign of so much building that hinted legend is heaped upon hinted legend. It is such a huge creation it has kept nearly 5 generations of fans amused, creatively building and extending, with languages, smaller scale works and even the equivalent of the Sealed Knot Assosciation, all based upon Lord of the Rings.
In this field Tolkien is, to the best of my knowledge, quite unique. The only other author I can think of who has created such a generally inspired work is Larry Niven with Ring World and Man Kijin Wars sagas.
Surely that smacks of jealousy?
Any author would give his typing finger to create a work that still has enough royalties coming in 30 years after his death to keeping him in tea and biccies, wouldn't they?
Terry Brooks is a good writer, but I do not think he is up to that level of depth, even if he had the desire to create such a thing.
Next and more likely thought was Terry Pratchett and his Discworld series, which is building in all sorts of odd places and fits his description almost exactly. Again Pratchett has his own style of writing that Brooks cannot hope to compete against, but as the two are exclusive of each other there seems little to complain about.
It could also be a side swipe at television shows (Star Trek, Star Gate, etc), that have inspired a huge range of side writing, both professional and amateur, that try to add the cultures and depth that are totally missing.
The final and hopefully most probable was that he was just trying to forestall some of the stupid questions that fans inevitably come up with that he had not thought of and are as he observes irrelevant.
Having said that, it does offer this as some very good advice for aspiring writers:
A good novel consists of four central elements: theme, plot, characterization, and style. To the extent which an author holding valid ethical values understands these elements and is able to properly integrate them, the book will be enjoyable because it presents a sense of life that many readers intuitively recognize as valid.