If you have access to an AS Maths textbook it should be there under C2.
I don't know if your confusing this for something else, but turning points is about the discovery of the electron, wave particle duality and Eintein's Special relativity.
The unit 4 book is quite good; I find most of the explanations are better than the AQA textbook (bar energy of a system undergoing SHM). Both books have practice exam style questions at the back (with answers and explanations), which are useful.