Read this on holiday. I thought I didn't get the ending at first, so I consulted the I Google and discovered that nobody else really did either. Some people have their own interpretations; here is mine.
Spoilers spoilers spoilers!
The most widely accepted explanation of the flat ending is that Philip K. Dick cast a hexagram of the I Ching whenever his characters did, and continued writing their actions based on what the hexagram said, but that at the end, the I Ching just clammed up and wouldn't give him anything useful. Hence the flat "that's it?" ending. I can't find a precise reference for this, so it may not be true.
That's why the ending is what it is, but what is it? I have seen claims that the Inner Truth reading tells us that the world of tMitHC is not real, and neither is ours, but that of tGLH is. I thus continue this line of reasoning to conclude that tMitHC is not a book about history or war, but a science fiction book whose single proposition is "what would a world be like if the I Ching really worked?" PKD shows us that this world underlies the 3 implied shadow realities in tMitHC: the reality of the narrative, the reality of tGLH, and the reality of the reader. The I Ching being true implies that tGLH is the true reality, implying that ours is false, fitting in perfectly with the fact that the I Ching doesn't work in our reality!
Well that's my pointless tautological theorem out of the way. The meat of the book is pretty much what you make of it. Just read it and say to yourself "Yeah, yeah, it's about reality. Gotcha."
It's thoroughly worth reading, if that wasn't clear above.