Prior to the recent publication of this book, there were two 'complete' [no book can be, but they make a stab in that direction] guides to the fungi of Britain. These are "The Encyclopedia of Fungi of Britain and Europe" by Michael Jordan, and "Mushrooms" by Roger Phillips. Both books are excellent large-format paperbacks that are ideal for identification purposes except in one respect - they are fairly large and heavy.
The purpose of this book in the face of two such excellent guides then is to provide something more of a 'field guide' that can be carried around when fungus hunting. This book is much larger than the 'Gem' guides' but much smaller than the two books above. It generally covers the same number of species as the books above, but in a smaller size.
So what's missing? Well, the textual descriptions, while abbreviated, are more than sufficient for identification purposes, the main omission being descriptions of what pores look like under a microscope - not something most readers will investigate. Apart from this, however, a more noticeable omission is whether species are edible, inedible, poisonous or deadly. This has long been a fixture of fungus enyclopedias, but you won't find it here, except for a few key species. If you want this information, buy the Jordan or Phillips books.
So the text is fine, what about the pictures - a book like this will usually be used by flicking through and looking at the photos. The photos in the book are of good quality, but they are all taken of upright fungi, and generally do not show the features of the stipe (stem) and gills/pores. Unfortunately without these gill/stipe shots it won't be possible to positively identify many species, and you will have to check another book, or search for images on the internet. The larger books, mentioned above, both have the space to provide more photography, which will make positive identification much more likely.
So in conclusion, the Jordan and Phillips books are the best fungus guides on the market, but if you are looking for a smaller 'field guide', this one is definitely the best available. Just don't be too disappointed if it turns out to be tricky to actually identify the fungi exclusively using this book.