1. I heard "Twitter" is a Ruby application. What are the other major applications Ruby has been used?
Twitter is a Rails application but they've started to use Scala a lot for things like message queues and heavy lifting in the backend. Other big apps using Ruby (and sometimes Rails) include Scribd, YellowPages.com, 43things, Penny Arcade, and all of the 37signals applications like Basecamp). Other companies use Ruby in the backend too, such as Amazon.com. See this list
of some companies that use Ruby/Rails.
2. What is the advantage of Ruby, compared to other languages? What are the other languages similar to Ruby?
This has been covered a few times in answer to other questions, so please have a look through the last day's of posts in this forum, but.. to summarize.. Ruby isn't verbose (if you consider Java to be a 7 out of 10 on the verbosity scale, Ruby's perhaps a 2) and it "gets out of the way" and lets you simply code your logic and features with as little architectural work as necessary (of course, if you want to, you can write Ruby in a very anal way, but it's not necessary to start with). I'd say Python is quite similar to Ruby in some ways, but it's not quite as dynamic and consistent. Ruby takes a lot of influence from Smalltalk, so that's going to be quite similar too ;-)
3. What does your book cover, which other books do not cover?
I'd say it's not so much about the "what" but the "how." Beginning Ruby does cover some things other books don't simply because it's trying to get someone from knowing very little to being able to "talk the talk" and have a good feel for what's involved in a modern Ruby developer's day to day work. For example, SQL, network daemons, the daemonization of processes, RSS feeds.. these things are all covered at some level. Other Ruby books tend to focus solely on language mechanics. Beginning Ruby also has a chapter all about the history of Ruby and Rails and how the community works and how you can get involved. Again, other books don't look at these "soft" issues much, but I think they're important to learn to feel part of the community.
Generally, though, Beginning Ruby is aimed to cover as much as possible but without confusing or rushing the reader. This is a hard balance to get right, but explains why the book comes in at almost 700 pages. It's not a pile of waffle like in many books, we actually need that many pages to cover all of the ground at the right pace!
4. What job and commercial prospects does Ruby has?
A couple of years ago it was pretty crazy. You could get a pretty good job (or freelance gigs) by just saying you know some Ruby or Rails, but now it's a little harder. One problem is that a lot of the Ruby gigs going around now are for senior developers and it's hard to find positions if you're just starting out. That said, Ruby and Rails people get pretty good rates (on a par with the Java world, I'd say - and certainly better than for PHP developers) but.. there's definitely an expectation of community participation and a history of open source projects or contributions from a lot of companies in this field.
If I were a developer using another language, say Java or C#, and wanted to get into Ruby, I'd do it on the side. Get involved with some open source projects, and try to get some smaller gigs on the side, before even attempting to go into it full time.