Starting Forth, by Leo Brodie, is a great book for beginning in Forth. It's very dated, but still excellent in most ways, and humorous. Unfortunately the only versions I've seen online were where apparently someone just re-keyboarded the whole thing, without the pictures, the bold and italics, etc.. The Forth Webring's address is http://t.webring.com/hub?ring=forth . You'll find a lot of good websites linked in there, including tutorials.
That was the book I started into Forth with, with the HP-71. Unfortunately the situation I ran up against was that the book assumed you had something to practice on, and the HP-71 Forth/Assembler module manual assumed you already had experience in Forth and only needed to know the things that were particular to the HP-71. The two were left pointing at each other. It wasn't ideal. In your process of learning, you're welcome to E-mail me with questions. I haven't done any Forth programming on the 71 in quite awhile so I'll probably have to do some review, but it'll be good for me! The 71's Forth implementation is not nearly as good as its BASIC implementation (especially when you include the user groups' LEX-file and other contributions to the BASIC); but since it's Forth, you can make a lot of improvements yourself, even without assembly.
After you get some beginning experience in Forth, another book that's good is Thinking Forth, again by Leo Brodie. He has relicensed this book under the Creative Commons license. Although unlinked to in the main page, you can get a copy of the Thnking Forth book at the following URL:
http://thinking-forth.sourceforge.net/thinking-forth.pdf
The pictures' quality is not very good, but it's all there, more or less in its original form.
Garth