I bought this book way back in 1979-80 along with another book published by Futura Publications Limited. I'll talk about The Book of Alien first, the first few pages give details about where O'Bannon got his idea for the story from, what he was doing prior to writing it (namely working on a version of Dune which didn't work out leaving him living on his friends (Shusett) sofa and needing to get back to work), and how the major players, Ridley Scott, Brandywine (Gordon Carroll, David Giler and Walter Hill), as well as Giger, Ron Cobb and other designers came to be working on the film. So it does give a brief idea of how it came about, however the book is primarily a visual record of the art work, from the Nostromo (interior and exterior), the costume's, and of course Giger's work on the planet, derelict spaceship, the space jockey, and of course the 3 versions of the Alien itself.
Quite frankly I love this book, I love looking at the photos and paintings, to see the imaginations of these people at work. Well worth buying in my opinion as I love all this sort of stuff, makes me wish I had this kind of imagination. So if you love these sorts of books then really worth getting.
I've also purchased several other books over the years, one is Giger's Alien, was a little bit expensive at the time but again well worth it just for his designs and paintings. The man has a truly bizarre, frightening but incredibly beautiful visual style, and his designs for the film are incredible. The second book by Futura was simply a movie novel, it took stills (1000's of them) from the film and turned the film into a book, if you can find a copy of that then also worth getting.
The last book was by John L. Flynn called Dissecting Aliens Terror in Space, where he writes about ALIEN, ALIENS and ALIEN3, giving in depth accounts of the production of each film in turn and worth picking up if you can find it.