The site says, "Available in digital form only (epub, kepub, mobi, pdf). Includes separate books for JavaScript, PHP, Python, and Ruby languages, and beer and milk beverages."

There is no mention of needing special software to read them, so I think it's safe to guess that there is no DRM. And it's sold directly by the author. Publishers are generally the ones who insist on DRM. It would not surprise me if there was watermarking, but that is not DRM.