This is the standard reference for first edition identification in the used book world. McBride guides paraphrase Zempel and cannot possibly be better because the Zempel guide reprints information obtained directly from the publisher. To quote from the Introduction, "Every statement, however, was prepared with the full cooperation and participation of the publisher." Unless it is believed that publishers want to intentionally mislead readers, this is the best possible guide to identifying firsts.
The guide is very easy to use. Publishers are listed alphabetically and their method of identifying first editions organized by year. Publishers change their practices over the years, so a Scribner first edition from 1924 will look different on the copyright page than a first from 1935. There is also an incredibly useful section on identifying book club editions which would solve many problems in identification, particularly in the cesspool of errors known as eBay. This book is built upon a guide to identifying firsts which originally appeared in 1928, using the same methods. The quality, quantity, pedigree and specificity of the information is second to none and there is no substitute for this book.
This is a reference book for the office, so the McBride guides would be a decent supplement for hitting the road. Also, this relates the practices of the publisher as obtained from the publisher, so this book will not give you points (book term for specific oddities about a particular book edition) on printing errors or on separating issues of a dustcover. For that kind of info, you need a specific bibliography such as provided by the Ahearns or the FPAA by Bruccoli.
If you are in the book world and do not have this book, you are an amateur regardless of sales volume. This is where first editions are defined, so pick it up or risk returns and ridicule from savvy collectors. Better yet, quiz your bookseller to see if they are ignorant or informed about identifying firsts.