I'm not sure what sophomore-level was for you, but the next step or refresher would be Quantum Mechanics. Griffiths is the standard introductory text, though I'm partial to Liboff. You'll want to work through a higher level textbook of QM after that; Sakurai and Shankar are two popular ones.
From there, you'd have a good basis to study some particle physics. The two introductory texts are either Halzen and Marten or Griffiths (same author, different book). Either of those will give you a pretty solid understanding of particle physics, including QED. You could supplement that by studying some Quantum Field Theory (QFT). At the moment, Peskin and Schroeder is the classic text.
Depending on your interests, you could continue in QFT with P&S. That text probably covers at least 3 terms of graduate QFT. You could also study GR. Carroll is the most popular text at the moment, but I like Schutz and others swear by Wald. If you're interested in experimental stuff, you could learn about detectors using the text by Green.