HTML5 Foundations by Daniel Foreman
HTML stands for Hyper-Text Markup Language, it was invented in 1993 by Tim Berners-Lee it was invented by CERN (makers of the large hadron collider). This revolutionary language allowed scientists to link between documents, allowing them to cite sources, provide supporting material and link to other works they wrote.
Today HTML is used for the same reason! Google search engine displays results in HTML and thanks to it's Hyperlink ability (the blue links you click on) the results will allow you to discover new content.
Since the original HTML release, it has become the standard for creating web content. It has gone through five major revisions, and is the single most important language to understand if you want to do anything on the internet.
HTML5 Foundations will teach you everything you need to know so that you too can interact with a huge community of web designers, developers, and other professionals.