The next step book wise is Arora and Barak, Computational Complexity: a modern approach. Of course that book is already out of date. The breakthrough that comes to mind is Ryan Williams' circuit lower bounds. He wrote a nice explanation of the proof here: https://arxiv.org/abs/1111.1261
He defines all the key terms pretty well but probably requires a bit of "maturity" to understand. There are also various breakthroughs to do with interactive proofs.