The Insight Guides come in many forms, but this one is exactly what it says on the cover, a pocket guide, and in that guise I think this book is very useful. The Insight Pocket Guide to Cairns & the Great Barrier Reef is only about 100 pages and is nice & slim (5mm thick) to take on your travels. I lived in Cairns for a year & used it frequently when I had visitors or wanted to explore on a day off from work. My parents (who would live in Cairns if they could) also take it with them on every trip.
The book starts with a 10 page history & culture section, which is quite brief & generalised before following the usual Insight Pocket Guide formula. The book contains many maps of the area of varying standards, but you would be wise to get a road map of the area if going outside of the city.
In the first section of the book there are 8 "itineraries" set in & around Cairns, the first takes in the North end of the city, the botanical gardens & RFDS base, whilst the subsequent ones take you slightly outside of the city to Kuranda (including information on the Train & SkyRail), walking around the rainforest & Port Douglas & the Tablelands. The information provided is functional & useful (i.e. phone numbers, websites & opening times) but not hugely detailed; I do think that is mainly because you will learn the background detail as you actually visit the places. It does give you addresses & numbers for visitor centres etc for further information as well as a few restaurants to visit, but it is NOT a comprehensive guide. The itineraries are very descriptive & give you clear directions to follow & advice on what to take with you & how to get the most out of your day.
The second section comprises 5 "excursions" - places reachable from Cairns, which is less useful in my humble opinion. For example it gives a few pages over to the Whitsundays, which is realistically a full days drive & offering only 1 company as a guide, however it does advise of some good stopping points & eateries along the route south.
My only other comment is that the title of the book is perhaps a little misleading as there is a wealth of information on the Great Barrier Reef out there - enough to fill this book by itself, with all the different dive companies & boats etc, whereas this book only describes the Quicksilver snorkelling/diving day out. If you are a diver, I am sure you will have done plenty of other research on dive sites, rather than just relying on the pocket book.
This book should be used in conjunction with a travel guide covering a larger area (e.g. the big Australia guide book, or Queensland & the GBR (lonely Planet) - however I personally found it invaluable for exploring the area local to Cairns & it is small enough not to cause too much distress to whomever must carry it around!