The first step required in any electronics project is to obtain datasheets for all of the components. Whenever
this book makes references to "
the datasheet", it is referring to the official datasheet
for the ATMega328P. The datasheet is available on the
Microchip
site. Specifically, this book was written against the
DS40002061B datasheet (which
contains Microchip branding as opposed to the older Atmel branding).