I don't think this is the only book about Mars you will ever need. I was disappointed with it for several reasons. Though I think it's well out of date now but I have a few points to make about the book.
The authors seem hooked on the theory that the Viking probes that landed on Mars in 1976 found evidence for life. This has been discredited very long ago (and increasingly discredited with all the new discoveries being made about Mars) and is now only advocated by essentially one person (Gil Levin now in his 80s) who is pathologically and irrationally wedded to the idea. The book is scientifically unbalanced. It is also full of clichés about Mars that a cursory level of research would have found out were untrue. They say the Saturn Five rocket on its side at the Kennedy Space Center could fly to Mars tomorrow if it was fuelled - what nonsense.
But more than this, the style of writing of the book irritated me. It is written in a sub A-level standard of prose with no flare, intelligent use of language or narrative sense. The books structure is all over the place. In one place (page 13) the authors actually use the word enormity as though it meant "an enormous thing." I was puzzled as to why, after so many years writing books, the two authors cannot write and then I realised that for many years they have not written an adult book, just a smattering of words for children's book that are mainly illustrations.
The conceit of the authors irritated me as well. The text is full of themselves, making one suspect that this is their favourite topic not Mars or astronomy. They say they are sitting on a veranda having drinks, jetting off, called by the BBC's science correspondent, etc, etc...enough, enough. If the authors want to write like that they need considerably more skill to pull it off.
In short - nice pictures, amateurish text, poor research, out of date.