You could read ICAO document 8168 volume 1 and 3, it explains the theory behind instrument procedures. Still does not teach you the principles about IFR flying though it really is a combination of multiple sources, most flightschools I have worked for have their own “Instrument Flying Study Guide” where they combine most of the knowledge in one book.
- ATPL Airlaw covers the important aspects of doc. 8168.
- ATPL Instrumentation covers working of instruments needed for IFR.
- ATPL Radio Navigation covers the working of beacons, GPS, radio’s, transponders etc.
- ATPL Human Performance and Limitations about disorientation, sensory illusions, also required knowledge for instrument flying.
- ATPL Operational Procedure explains weather minima for operators, RVRs, ceilings for precision and non-precision approaches and alternates.
I guess the above includes most of the required knowledge for instrument flying although there might be other subjects with an overlap. There is little bits scattered through all the 14 ATPL subjects I’m afrand. There isn’t really one book.
I have started writing something like that during my COVID not flying period and convert all the lesson materials and manuals I ever wrote into one presentation but the IFR part isn’t finished and I’m flying again now. But if I ever manage to finish it I will drop it in the forums, its nearly 800 pages already describing basic VFR flying and procedures + navigation, IFR (work in progress), ACAS, TAWS, flying in NAT airspace, RVSM, performance, Radiotelephony (see my ATC mod, I published the RTF part with that) all the codes and tables needed for flight.