All the resources are on Infocenter and in Tutorials section. Once you are familiar with basic concepts (if you intend to use any radio - like BLE - then you need to be familiar with its stack and principles) then you will most probably use just three documents:
Chip Product Specification (e.g. nRF52832 or nRF52840)
nRF5 SDK API (and related) documentation
Radio stack specification/description (if you use any - for "Soft Device" stacks released by Nordic they also release Soft Device specification, see Infocenter link above)
