The parts on the working holidaymaker (what the Youth Mobility Scheme was called ) is very small and I am not sure of its accuracy now. The book was published in 2008 so there can be a lot of changes made to the system since it was published.
The first part mentioned about the scheme is this:
"The working holidaymakers scheme allows young Commonwealth citizens the opportunity to come to the UK for an extended holiday which they may help fund by working in the UK up to a maximum of two years. To qualify you must be a Commonwealth citizen, be aged 17 to 30 inclusive and either be unmarried, or married to a working holidaymaker who is taking the holiday with you.
During the period of stay, you may take employment incidental to a holiday but not set yourself up in or run a business. You can choose when to work and when to take your holiday breaks, but you are not allowed to work for more than a total of 12 months out of the 24. You must also be able to prove you have enough money for your accommodation and living expenses for the first two months after you arrive, or for at least one month if you can show that you have arranged a job in advance."
So I am not sure about the whole working only 12 months of the 24 because I have not read that anywhere else. I think this might just be old information.
The website for the UK Border Agency states that "You will be able to live and work in the UK for 24 months under the youth mobility scheme."