This book is a companion guide to the main "Field Guide" (Field Guide to the Moths of Great Britain and Ireland), and is now in its second edition, with updates in line with the main Field Guide's third edition - in particular recent changes in taxonomy and species added to the British fauna since the first edition.
The Concise Guide is a superbly practical condensed version, more suited for taking out in the field and comparing live moths against the illustrations. The book is essentially the full set of colour plates from the main Guide, with shortened species descriptions alongside each plate. Richard Lewington's marvellous illustrations are of moths depicted in their natural resting position (with occasional extra illustrations such as hindwing detail, antenna enlargements or other identification pointers). This makes it easy to leaf through a section (or even the entire book) to compare a live specimen against the plates. In addition, the ring-bound format means the book can be laid flat with the plates open, and doesn't spring shut like the main guide does, just at the wrong moment!
Due to the compactness, the species descriptions are necessarily shortened, and contain a number of abbreviations describing status and distribution, habitat and so on. At first these can be a little daunting, but it doesn't take long to grasp the concepts, and these soon become second-nature.
Overall, this is not only a great companion to the main Field Guide, but is a very useful book in its own right. It particularly stands out if you happen to have a potted specimen that you're looking to identify - the life-size paintings and flat layout allow you to compare your specimen directly 'on-page' next to the likely suspects.
The book is around half the price of the full Field Guide, so if you're on a strict budget, but are looking for a comprehensive field guide to Britain's moths, you can't go wrong with this.