The site came first — lawsofux.com, one card per law, a poster's worth of design on each — and the book is the site with the reasoning filled in: where each principle came from, what the research actually showed, and what a designer does about it on a screen. Thirty entries in the site's current list, from the Aesthetic-Usability Effect to the Zeigarnik Effect, grouped by whether they describe perception, memory, attention, decision-making, or how people actually behave with a product.
Read it as a vocabulary for design reviews: "that's Hick's law" ends an argument about a menu with forty items faster than a paragraph does, and "Jakob's law" is the one-line case for not reinventing a pattern the user already knows from every other site. Several of the laws are glossary entries waiting to be written here — Fitts, Hick, Jakob, Tesler and Postel first, since each names a rule this site's components already follow or break.
The site is free and is the better daily reference; the book is the one to read once, start to finish.