The opening to this book is an odd way of doing things. Stephenson overwhelms the reader with neologisms and ceremonial details that could be off-putting. It's worth fighting your way through though because after 50 or so pages, the talk of auts, apert, theorics and itas, dies into the background and the real story begins.
Erasmas is part of a concent, a place that holds scientists and mathematicians known as the avout in perfect isolation from the Saecular world, until Apert, when the two worlds can intermingle. The intermingling does not always go well but ends after ten days allowing the avout to go back to their reputedly better world. But something else is happening, there's a rogue star in the sky that may represent the need for a massive paradigm shift in how the universe is seen and soon Erasmas has to leave the concent, perhaps forever, in order to save his world.
Along the way ideas are discussed that you'll probably recognise if you've read any Plato, Kant or Philip K. Dick. If you already have an interest in the nature of reality you probably won't find anything new, but that's okay, because Erasmas is a fine protagonist to travel with and there are enough ambiguities and incidental ideas to keep you interested. As ever with Stephenson the kitchen sink is in there, too, but he does it all with a light touch and a sense of humour that allows you to get comfortable.
At the end is where it all goes a little wonky. I can't give away too much but there is an application of thought experiment to reality that undercuts the story rather than illustrating its points. I think it was an effort to create a bigger pay-off, but in the end it feels a tad too mystical in the face of all that has gone before. Had it been brought in a little earlier in the narrative it might have felt less forced.
Despite this flaw, I still think it a fine book, but those new to Stephenson should try his earlier works first.