Not entirely sure what your asking in the first question. The wave equation is a really important part of AS physics, but rather than just tell you that this equation is true, the book is explaining where it comes from and why it is true. It simply starts off with the idea that speed = distance / time, and then says "What if distance = one wavelength? This makes the time = the period of the wave." So, in this case, speed = wavelength / period, and as the period of any wave is 1 / frequency, this gives the wave equation; speed = frequency x wavelength.
The important thing to remember for A Level is that, unlike most equations, you are expected to be able to derive the wave equation in the above manner in an exam.
If you're actually asking something else, do say so, and I'll have another go!