As someone who recently did that after ~8 years, I found it useful to skim through Modern Java in Action. It should be pretty easy read if you've been doing more modern languages, but important to learn Java semantics and nuances. Also, I highly recommend Java Concurrency in Practice, but it's a tougher read.

That said, for most orgs, the biggest changes you might see would be in libraries and frameworks used. There will most likely be less XML, better build tools and more modular library usage than 12 years ago.