There are at least two really good books for Selenium beginners, and quite a few courses/tutorials out there, as well as a lot of blogs.
However, I haven't seen quite as much out there for Watir - I'd like to know about useful resources for:
Watir or Watir-Webdriver for those new to using it, but who have some automation experience
Introductory resources for testers who haven't done automation before
In terms of introductory resources, I'd recommend the book 'Everyday scripting with Ruby' by Brian Marik it's a great into to scripting/programming with ruby, and since everything watir does is powered by ruby, getting a good basic grasp of the language is important. Also the book is very much 'aimed' at testers.
The other thing that's essential (for anyone doing ANY kind of webtesting) is a basic fundamental understanding of how HTML works, the various objects and tags, and how web pages are structured. There's probably a dozen online tutorials you can hit that will teach the basics of HTML. Myself I mostly self taught it via taking web pages apart and using 'webmaster in a nutshell' but that book (a great reference IMHO) is now out of print and a bit long in the tooth.
For Watir itself, the Watir Wiki has a tutorial http://wiki.openqa.org/display/WTR/Start+Here which I recommend as a first step/introduction. Also essential is the Watir RDOC which is the standard way any Ruby 'Gem' is documented http://rdoc.info/gems/watir/frames which is basically your technical reference (if you want to see what methods are available for given elements etc.) You'll find links to all the above on the "Documentation" tab of the main watir.com website http://watir.com/documentation/
The best place to ask for help once code is involved is IMHO Stack Overflow (just tag the question with the 'Watir' tag) since it allows better formatting of sample code, page code etc. A close second is the Watir General group in Google-Groups (http://groups.google.com/group/watir-general) which is a very active watir community (although the restrictions on message formatting make it less useful for support)
The watir.com website is a great collection of resources about Watir. Željko Filipin, Watir community manager, is currently working on a book about Watir (https://github.com/zeljkofilipin/watirbook/). It is currently a work in progress, but I'm sure he would very much be interested in hearing what kind of things someone new to Watir would like to see in the book.
For watir-webdriver there is http://watirwebdriver.com/ site created by Alister Scott.
