After years of insisting that Ivory soap’s ability to float was the product of a production error, Proctor and Gamble is now conceding that it was in fact due to a calculated marketing effort. This is just a one of the dirty little secrets revealed in Rising Tide: Lessons from 165 Years of Brand Building at Procter & Gamble, due out on July 8.

Of course, Ivory wasn’t the first floating soap, it was merely the first soap to actively promote its floatiness back in the 19th Century with the tagline “It Floats,” which may well be the longest continually running tagline in existence. What keeps this book an edge-of-your-seat page burner is some of the newly discovered internal documents from the P&G archives. For instance, here’s the one that nailed them on the floating controversy that has been dogging the company for 125 years:

Ed Rider, P&G’s company archivist, said he has discovered a notebook entry from 1863 by P&G chemist James N. Gamble, who wrote: “I made floating soap today. I think we’ll make all of our stock that way.”

Gripping stuff. If you can’t wait a whole two weeks for the book to come out, the Cincinnati Enquirer has more sudsy bites right now.