Many older books, particularly from 1890-1930, include several pages of advertising at the rear. These ads are usually for other titles available from the publisher but sometimes they’re for products and services.
A bookseller should always read through these pages because they often contain useful (or at least entertaining) information. You’ll get a sense of the publisher’s output and discover books you never knew existed.
In children’s series titles, which have minimal copyright info (particularly from Grosset & Dunlap), the ad page is invaluable in determining the printing of the book. Plus you can look at the high and low numbers of others series (always the most rare) and keep your eyes peeled for them.
Lastly sometimes the ad itself is the most valuable part of the book. I’ve found early ads for Arts & Crafts furniture, reading appearances by celebrities, and first announcements of important books that have been the selling points for otherwise lackluster items.
William Smith of Hang Fire Books