HTML itself is very simple. It's one of the easiest things you can learn. But it won't take you all that far in web design. Essentially, HTML is a set of tags that you use to tell a web browser what kinds of contents you have. for example, you can specify which text should appear a standard paragraphs, which as headings, which as list items etc.
But HTML on its own is very plain, so at a minimum you need to learn CSS as well. There are many books on HTML and CSS. Both are—in essence—very easy to learn. You can be up and running in days—which you can't say of too many subjects.