I read a great book at my local OPC - https://www.amazon.co.uk/Porsche-70-Years-There-Substitute/dp/0760347255
This and this new one.
https://www.amazon.com/Porsche-70-Years-There-Substitute/dp/0760347255/ref=sr_1_1?ie=UTF8&qid=1533160809&sr=8-1&keywords=randy+leffingwell+porsche