Having mastered the principles of physics, you now possess a strong basis upon which to study more complex and specialized subjects, such as (but not restricted to): 

• Quantum Field Theory: Quantum Field Theory (QFT) is the heart of all modern high-energy physics: the Standard Model of particle physics is a QFT. The whole idea behind QFT is that we are doing quantum mechanics on classical fields, and it works remarkably well. Along with GR, QFT will be the most challenging part of your physics education, but perhaps the most rewarding (I know it was extraordinarily rewarding for me!). It may take many, many years to master. I recommend to studying: Quantum Field Theory in a Nutshell by by Anthony Zee and An Introduction To Quantum Field Theory by by Michael E. Peskin, Dan V. Schroeder.

• Electronics. The field of electronics is a branch of physics and electrical engineering that deals with the behavior and effects of electrons using electronic devices. You shough learn very basic things about electronic circuits also. I recommend studying Basic Electronics for Scientists and Engineers by Eggleston.

• Special and General Relativity. I recommend studying Concepts of Modern Physics by Arthur Beiser, Spacetime and Geometry: An Introduction to General Relativity by Sean M. Carroll, and Einstein Gravity in a Nutshell by Anthony Zee.

• Astronomy and Astrophysics. I recommend studying Cosmic Perspective by by Bennett, Donahue, Schneider, Voit and An Introduction to Modern Astrophysics by Carroll and Ostlie.

• Biophysics. I recommend studying Biophysics: An Introduction by Glaser.

• Particle Physics. I recommend studying Introduction to Elementary Particles by by David Griffiths (Undergraduate) and Quarks and Leptons by Halzen and Martin (Graduate).

• Quantum Computing. Quantum Computation and Quantum Information by Michael A. Nielsen and Isaac L. Chuang. 

• String Theory. I recommend studying A First Course in String Theory by Zwiebach. 

• Other. Nuclear Physics, Radiation Physics, Atomic Physics, Semiconductor physics, and Plasma physics.