When was the first PHP book published?

As far as we know, the first books dedicated to PHP covered PHP 3.0. The very first of those was a Czech book by Jirka Kosek published 20 April 1999 entitiled PHP - tvorba interaktivních internetových aplikací (direct translation: “PHP - Creation of Interactive Internet Applications”).

May of that same year brought the German publication PHP: Dynamische Webauftritte professionell realisieren (direct translation: “PHP: Realize Professional Dynamic Websites”) by Egon Schmid, Christian Cartus and Richard Blume. The first book in English on PHP - Core PHP Programming by Leon Atkinson - was published soon after.