It is important to go into this book with an open mind.
For me, the entire premise seems to exist outside of the realm of fiction and nonfiction, as both of these genres connate a certain type of book. Communion is neither. It is to be read and absorbed, not analysed or debated for its veracity. The premise of the story, for me, exists outside of what is "true" and what is not.
Rather than being a story about an alien abduction, it is much more about Strieber's own journey of realisation, discovery, and healing. The reviewers who say that this book frightened them, I can see why: The first fifty pages or so are fairly frightening. However, the other threehundred-odd pages deal with Strieber's grappling with how to heal himself, how to deal with what he had perceived to have happened.
It is important to read this book not as a science fiction or science fact book, but rather as a man's struggle to heal. I found some parts of this book to be slow and occasionally confusing. Additionally, Strieber's writing can be difficult to follow and repetitive. Finally, he tends to get a bit too "new-agey" for me at points. Despite this, however, you cannot help but to feel the resurgance of hope, the renewed faith and tenacity which he experiences with his facing of the unknown.
This book is not meant to be frightening, it is meant to be a testament to inner strength.