And that's why I give it three stars (although really that's more of an average between four and two, depending on who you are and what you're after). If you're a child, or someone without much of a background in science (particularly physics), and if you're interested in learning, then this book will be interesting and I think it's well worth a read. If you, say, have a physics A level, then a good deal of this book will be covering old ground, and at a pace you may find slow. If you are after something to whet your appetite while Pullman slaves away on his next installment (aren't we excited!) then this is probably not it.
Let's be fair - it's not fiction, and it's not Pullman, so you're not likely to be glued to pages. But it never claims to be. What it is, it does well, the science in here is detailed without being overwhelming, and fairly complete - to be honest you barely need to know anything at all to be able to understand the science here, you just need to be willing.
And it does show what I feel it was intended to - that a lot of what Pullman wrote about in His Dark Materials is based (on some level) in fact. And that is of course what makes Science Fiction writing so exciting and so addictive - and also what makes studying science so fascinating for science fiction fans. And it's good for the imagination of the romantically inclined, like me - the fact that the existance of parallel worlds is hypothesised at the highest levels gives me that little chance to believe that His Dark Materials might all be true...
But anyway, the book itself is a good, easy read to dip in and out of or to rattle through, and hopefully this review has given you a clue as to whether it's right for you!