Java Concurrency in Practice

Java Concurrency in Practice is basically the bible for concurrency and multithreading in Java. However, it is not free. If you prefer free, the one that I used to learn was the Official Java Tutorial on Concurrency, provided by Oracle itself. However, this one is outdated. Nothing in it is wrong, but some of the suggested strategies aren't necessarily the best ones anymore. Again, none of them are wrong. Finally, if you want up-to-date resources that are trustworthy and effective, I would start with this video. I haven't watched this video myself, but John tends to have reliably good content, so I'd trust it as a good intro for beginners.

A popular book recommendation is Java Concurrency in Practice by Brian Goetz