Harry's Lattice. The Atharrais Sequence, #1

Imagine if you got home one evening, unlocked your front door, opened it and went into your hall; only it wasn't your hall. Imagine it was completely different to anything you had ever seen before. Not a hall at all, but a dizzying tableau of clouds, sky, ice fields covered in dazzling snow; all seen from the top of a huge mountainous Himalayan peak.  Just inside your front door. Or imagine you were faced with an alien landscape, maybe from one of Saturn's moons with Saturn himself and his rings low against the horizon against an inky black sky.
 The temptation would be to close the door again without stepping over the threshold; give it a moment, then open up again in the hope that the unwarranted intrusion of craziness would have gone away. Harry lived his life and went about his normal business, unaware as the rest of us that nothing is real. An impossible meeting uncovers the truth and introduces him to the true nature of the world we take for granted.
But awareness of the truth is rationed, for a good reason. Harry's world soon turns upside down and he must rely on new friends and acquaintances to help him make sense of it all. Drawing on several current scientific ideas, Harry's journey explores his personality; illuminating the experiences that form him as well as opening a door into a shadow-world of incredible concepts which seem crazy but which nevertheless underpin our world.