Actually, it is peer reviewed following the standard practice for books: some other people read it and provided feedback as evidenced by the Acknowledgments section.
The funny thing about books is that authors in free societies are allowed to self-publish whatever they want. The norms are different and, frankly, more democratic and with less gatekeeping.
arXiv is a preprint server trusted by the scientific community for decades - papers there often undergo peer review later, and many top ML researchers publish their work there first for faster dissemination.
