You learn best from doing. You could read every book in the world, but if you're not practising then your not improving.
If I were to learn HTML, CSS and PHP (which you would need to know if your going to mess about with Wordpress) then I would buy one of those books that promises to teach you something in 10 minutes. I did this, but I didn't read it cover to cover. I instead used it as a reference. The good thing with these types of books is that they cut out most of the rubbish and just give you a real world explanation with a short explanation.
I would also sign up to Twitter and follow different people connected with web design. Smashing Magazine for example. But even doing a quick search will give you lots of other web designers/developers who you could always tweet for advice.
The most important thing to learn, and to learn from the start, is best practices and standards. If you get your code to be standardised and cross browser compliant, everything else will come easy. There is nothing worse than learning to develop a website for just Internet Explorer, then realise people using Firefox can't see it properly. I learn this the hard way 10 years ago.
You are using the internet to develop websites, so the internet should be your first point of call. Do a Google search for HTML tutorials. These tutorials (for the most part) are free and easier to transport compared to a book.