Dr. Michael Heiser is a Christian and an expert in the languages and cultures of the ancient Middle East. A paragraph toward the end summarizes well his goal with this book:
"English translations fail to preserve nuances important in angelology, and popular studies depend on those translations. Little attention is paid to the wider ancient contexts of the biblical material, such as the ancient Near East and the Second Temple period."
Those deficiencies are exactly what he intends to remedy with this heavily-footnoted, academic work. This book continues several themes he regularly emphasizes in his writings:
One, the book could be seen as a sort of sequel to his book "Unseen Realm", in which he says "hey, all this supernatural-seeming stuff you read in the Bible that we tend to take as analogy or metaphor - the writers actually meant it literally, let's recover their supernatural worldview". In this book, obviously, the especial focus is on angels.
Two, Heiser is very much a "get back to the original text and grammar" kind of guy. He would probably call this an exaggeration (and perhaps it is), but he is anti-tradition to a fault (really, to a fault sometimes, I think) - traditions just make it difficult for you to see what the text actually says. On that note, the description of angels you will find in this book is richer than the usual Christian understanding, which essentially goes "angels good guys, demons bad guys". Heiser would say - there are many spiritual beings, and words like "angel", "cherubim", "seraphim", etc., are job descriptions, and there is much information to be found in the Old Testament that is somewhat neglected by Christians who focus much more on the New. In fact he probably would have preferred to name the book "Entities of the Heavenly Realm" or something but, sales would be less.
Third, and what will make some readers most uncomfortable, is that Heiser has no problem bringing in Second Temple literature and also literature from other cultures that he believes influenced the ancient Hebrews. And I am actually unsatisfied with... questions he raises that he never really addresses to my satisfaction. "What is up with that weird story in Jude about Michael contending with Satan for the body of Moses?" "Well, it could be reaching back to a Second Temple tradition, which the New Testament authors probably knew, which itself might reach back to this belief a lot of the ancient cultures in the region had that the particular piece of geography where Moses was buried was sort of the realm of the dead." OK but... the New Testament is inerrant, all that other stuff isn't, so should it bother us that you're suggesting the writer is pulling information from non-inspired sources? Is that piece of geography *actually* the realm of dead (I think Heiser would say "no")? If it isn't, should that trouble us in some way regarding the NT's inerrancy? (These questions don't seem to bother Heiser at all, but I do wish he addressed them head-on.)
So, an interesting book to read, though I would read it with care and skepticism (though I say that for all books). Aside from the concerns I raised above, I also think Heiser is sometimes overly confident about his own interpretation of a passage - read with skepticism, as I said. Organization of the book is mainly chronological, looking at Old Testament descriptions of the "heavenly host", then Second Temple angelology, then the New Testament. The book ends with what is essentially an "FAQ about angels" which some readers will find especially interesting.