Well, no, don't send money to such ads ever.
Announcing the publication of the book is one thing, asking to contact personally by email and pay $27 to receive a PDF which does not even have a Table of Content or a summary or reviews, number of pages, ISBN, etc. is a big no-no.

Johann Zietsman is rancher from Zimbabwe that promotes rotational grazing and worked with Elephant Man Allan Savory. It also uses very short rotations that require huge manpower, so I am not sure how it is relevant to large scale ranching, or how what works on the southern African veld can be adapted to the U.S..

Here's some information on Zietsman, from a Savory Institute ebook, probably "Holistic Management:"
http://www.savoryinstitute.com/media/38209/21-Zietsman-Zimbabwe.pdf
Here's a 2012 interesting article from Zietsman.

The ebook "Pastures for profit: A guide to rotational grazing" from the University of Wisconsin Extension is a great, free and recent introduction to rotational grazing. It also includes an "Additional reading" section at the end that references the old classics, such as André Voisin's "Grass Productivity."