It's a perfect book for new Christians or non-Christians. Its doctrine is mainstream, correct, and accessible to all. Most mainstream denominations --with the exception of Mormons and Jehovah's Witnesses-- will agree to all its tenets. Roman Catholics might quibble because such Catholic elements such as Purgatory and The Virgin Mary as co-redemptrix are not included. But, for the most part, the book shows the shared theologies of Christianity -- the belief in one invisible God who is uncaused and who caused all things, who works in and with humanity, whose Spirit collaborated with frail holy humans to create the Bible, and who sent his only perfect Son, Jesus Christ to reconcile mankind to God.
The book tells about Angels, Heaven, Hell, and the power of God. It is a good book, well written but also, curiously, "unnecessary" or even "old." It says nothing new. Neither does it say the old truths in a new way. Often many Christians do not read old Christian writers so perhaps it is a good thing for Christians to write a new book every few years to explain theology to new Christians. But for believers who have read their Bibles, there really is nothing new written in or spoken of or seen in Unseen...and the sections where one reads about the Christian view of salvation through Christ does make the reader wonder whom this book is written for.
Yet it is a good book and will no doubt bless many. I recommend this book.