Can you give us an introduction on the "Balsamiq Mockups"? Is it a mockup of only the web pages or is it possible to mockup all the GUI applications? What are the various types of mockups availables? What are the advantages of using "Balsamiq Mockups" over other mockups and maybe some disadvantages? Is there any reference materials such as books are available for these mockups?
The advantages of using Mockups over pen and paper is that you start digital, so it's easier to tweak and share your designs with the whole team, especially if people are not all in the same geographical location.
As for advantages of Mockups over other competing tools, we say that we like to compete on usability (we think we're the fastest, most fun tool to use) and customer service (we care about your success deeply). You can be the judge on how we're doing on these fronts.
Regarding books about wireframing, there are a ton! I would look for UX, User Experience, Information Architecture...it's really a vast field. If you need some book recommendations, we can maybe start a whole other topic on the subject.