This book covers all the species of seagulls that you're likely to meet in the northern hemisphere. It is very detailed, so if you for instance check "Herring Gull", you will find much more information about it than your regular birding book. Information about habitat, moulting, feeding behavior and even history is excellent. The different appearances of different age groups of seagulls are illustrated, not only in summmer plumage, but also winter, and what goes in between is described in the text. I don't find the drawn and painted illustrations particularly beautiful, but they serve their purpose. The photos on the other hand are really, really nice, and work well as illustrations as well, as there are so many of them. I see that many of them are by the author himself, and that makes the book a stunning feat by Klaus Malling Olsen.
I see that some reviewers describe the book as a "tough" read, but even though it contains some specialized ornitology terminology, I find it really easy to read. It contains information about birds that you are likely to encounter every day, and as such should be considered by anyone who enjoys birdwatching. Being able to tell the age of the big seagull scavengeing for chips that you pass on your way to work is oddly satisfying.