The FXRuby book from Lyle Johnson finally came out this month. I had an early preview/review of the book back in January before it went to the press. Even though I haven't much experience writing ruby code (I mostly program in C/C++), I think the book is quite easily to follow for non-ruby programmers. There may have been only one or two occurrences where I had to use google to find out what the ruby specific code meant. I guess this a compliment to the ruby language itself but also to Lyle in making the book easily accessible for us non-ruby people.
You can hear Lyle talking about book here.