Perfect for solving the tough ones,
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This review is from: The Macmillan Field Guide to Bird Identification (Hardcover)
As other have mentioned this doesn't cover all the birds you will find in the field but what it doesn't brilliantly is focus on those pairs or groups of birds that are similar or just plain difficult to identify and highlights the key features that are different.
A brilliant book that I will come back to again and again and superbly compliments the Collins Bird Guide - highly recommended