I looked at a few of Stepping Stones' books on Amazon and I'm not impressed, either by the books themselves (which are only available in Kindle format on Amazon), nor by their numbers. For example, an SF book released just two months ago has already dropped into the 3 million ranking territory overall. I tried looking for a few titles at Barnes & Noble, but they're not listed. Seems like the major outlet for sales is the Stepping Stones website. The paperbacks are fairly steeply priced, and when you add on $7.50 in shipping - well.
Stepping Stones only appears to have eleven books in their catalog, and it looks like most (if not all) of them were published in the last few months.
With no staff listed, that iffy "hybrid" option and a full page of "testimonials" from their authors, this looks like one to avoid.