There’s still a lot of useful information in it. Obviously, anything added after 2011 wouldn’t be covered. But, there haven’t been tons of major developer features added since 2011. The Customizer, REST API, and term meta (same as post and user meta, essentially) are 3 that come to mind.
There’s some solid info for building good plugins still in the book. It still sells fairly well, and we still get good feedback.
I’m a bit biased, but I think it’s a good purchase. Of course, I’m not going to tell you to not buy it. 🙂