Requirement for Developing

[ZX Spectrum]

Hello.
I would like to know what tools, books I need to learn in order to start developing apps for the PinePhone Pro ?
as a background about my experties, I am:
- very good at Java. (regular desktop applications)
- very good at SQL (probably irrelevant) 
- I am OK with Linux bash shell (no shell scripts though )
- very good with Operating Systems design concepts (theory only from university courses)
- no experience at all in user interface design (well I know Java swing at least)

I really appreciate detailed guidance and direct resources.
I prefer reading books over scavenging from badly written blogs  and tutorials

  thanks