Cheaper than books, try this website. It's free and has a lot of good starting points.
If your looking for inspiration and to see some cool design ideas/code, visit css-tricks. It's run by a guy called Chris Coyier and he usually posts some pretty interesting guides about web programming.
I highly recommend you consider HTML5 instead. There's no reason not to. XHTML is simply bringing stricter rules/standards to the markup which you can implement in HTML5 anyway.
Just because you may be hearing that some browsers dont fully support HTML5, that's OK. As you learn HTML5, you'll be learning about the new elements that HTML5 has... you are not required to use them at this time. You can stick with the standard HTML elements (minus those that are now deprecated).
I would recommend using the internet as your main resource. Personally, I find the books to be very mundane and you will not learn valuable coding techniques or be introduced to real world issues that may occur. It would be best to just do some research on W3C and check YouTube for tutorial videos. You will learn more that way and at a much quicker pace.
Personally, I find books very useful. The main reason is that they are peer-reviewed and usually well-structured, often leading you through a series of tasks as you develop skills. They also allow you to dip in and out at certain points, using the all-important Google page in the back.
Obviously, books date pretty quickly, so if you're buying, buy new.
With regard to web resources and video tutorials, they can be very quick to pick up. Unfortunately every idiot out there with a copy of Notepad and a browser thinks that they are authorities on web technologies... and so they post tutorials.
Note the links for 'this' and 'W3C' in the previous posts are for W3Schools. Just be aware that W3Schools are in NO WAY affiliated to W3C. They do not enjoy any special status. That is not to say that their tutorials are not useful, but be aware that they are just one site out of thousands. IMO, too much is made of this site as a 'great site to learn the basic on...'. They've sharpened up their act a bit, but they used to be error-strewn.
I am wondering if I could start incorporating HTML5 tags on pages that are HTML designed from 10 years ago or would it be best that I start using HTML5 to re-design all my pages. The website I am wondering about is www.usa-hero.com. I use a lot of <center> and <font> tags which I understand you don't use with HTML5.
if I could start incorporating HTML5 tags on pages
As soon as you change the document declaration to <!doctype html>, you just created an HTML5 page. It may not validate 100%, but that's the foundation for an HTML5 document.
If you plan on going back to some of your older pages, you can. With regard to deprecated elements and attributes, you can also start replacing those with the appropriate CSS.
Keep in mind that browsers do their best to render HTML markup. If the markup is written within the standard, there is a better chance that you'll get the same look and feel accross browsers. If not, they do their best to display the page as it was intended.
While center and font is now deprecated, most browsers out there will continue to render those elements as inteneded, but for how long? As new versions of browsers come out, the developers may begin dropping support for those tags.