"who's going to spend £22 when you can find more information with a quick Google search"
Hmm, there are plenty of sites with information, but I think this book is a good idea as it collates all the info you need into one place.
If someone needs to build a computer, chances are they do not already have a computer, or theirs is broken.
What exactly are you going to view this information on while you build your PC? Sure you could print web pages, but you would probably spend more on ink and paper by the time you had all you needed, plus it would not be organised as well and probably would be contradictory in places or have gaps.
No, I think this is a great subject for a book, if done well. IMHO, everyone should build their first computer - I had a mate teach me how to build my first PC and it totally demystified the subject and ultimately started me on my career.