What are good references to understanding the proof of the PCP theorem?
Both Goldreich's complexity textbook and Arora and Barak's complexity textbook have chapters devoted to explaining the proof of the PCP theorem (with pictures!).

Also, Dinur's paper is worthwhile to read, if you haven't tried to tackle it yet. It's at least more approachable (in my opinion) than the original proof, and you can get a good intuition for how the proof works by skimming just the first 12 pages (and delve into the technical proofs contained in the latter chunk of the paper later, if you prefer).