--Ursula K. LeGuin
Like so many other things in life--and literature--concern with accessibility is a tension rather than an absolute. Transparency is an ideal, of course--but then again, there are things it is not suited to. Things that can be handled best by telling our truths slant.
Things that can't be said because they have to shown, because language has limitations and one of those limitations is that it tends to be absolute. I talked a bit about this a while back, with regard to thematic arc.
It is not the job of a book to appeal to every possible reader. Specifically, there will always be readers who are too stupid for any given book, to whom it appears opaque or incomprehensible. And readers who are too smart for it, for whom it appears transparent and trite and obvious. (I am too stupid for much of crowleycrow's work, for example.)
And sometimes, the author is too stupid for the book. Or just stupid enough, as the case may be.
And there will be readers for whom any given book is just right. From a writer's perspective, of course--assuming that one is an honest craftsman and an honest artist, rather than a cynical hack--one writes to the best of one's ability the book which one needs to write, and once tries to make it accessible to the broadest possible audience while still maintaining the integrity of the book.
This is, as insinuated above, a balancing act. Every book has a sweet spot, I think--a place where it is as transparent as it can be to do the work it's trying to do. The more work, I would guess, the harder that sweet spot is to hit--and the more difficult the book may be, anyway, even in the writer pulls it off. (For the same reason that King Lear is a more difficult play than As You Like It.)
I'm thinking about this now because there's some unintentional opacity in By the Mountain Bound that needs training out, and it's hard work. Things that seem apparent to me may not be apparent to as many readers as they could be. Things that I would like to make apparent are not amenable to exposition or illumation without becoming heavy-handed, and are left in the subtext to sink or to swim. I mean, ideally we'd be beaming our Jesus sex thought rays into the skulls of every science fiction fan, but that only works for warren_ellis, as far as I can tell. So we're stuck with language, and if we're trying to write about things that are not amenable to simple explication, slippery difficult-to-consider things, things there isn't language for...
...indirection and example and interstices are what we've got.
The only advice I can offer here is for any given writer to hold to his integrity*, and remember--in writing as in baseball, just laying wood on the ball a third of the time is pretty bloody good. This stuff isn't easy.
*holding on to one's integrity does not mean proclaiming one's unpublishability as Misunderstood Genius. It does mean improving one's craft to whatever level one can and honoring the core of the story (I still believe accessibility is a literary virtue; it's just one that must sometimes be traded for other worthy but often inimical things, like depth and revelation), rather than selling your art down the river of Dumb for a fast buck.