"Nobody should be allowed to have anything to do with advertising until he has read this book (Scientific Advertising) seven times. It changed the course of my life." - David Ogilvy
Claude Hopkins is considered by many the father of modern advertising. He is credited with inventing or popularizing such marketing staples as product sampling, couponing, and preemptive advertising.
His two works, Scientific Advertising (1923) and My Life in Advertising (1927), now in the public domain, are most often found together in a single volume, as HERE.
I highly recommend the two-book volume. Hopkins' autobiography is packed with anecdotes, illustrations, and lessons that will profit anybody who sells, creates, or invests in advertising.
That said, the earlier volume is now available as a standalone, free download in .pdf format, here.