I've been doing Java on and off for a fair bit of time now. I feel very comfortable with pretty much all the topics you find in beginner books... threads, I/O, serialization, etc.
My question is, what should I move on to now? I'm hungry to do some more interesting stuff, but I don't know where to go. Are there any good "cookbook"-type books that follow some interesting Java projects? Are there other specific Java topics that are worth learning?
Any push in the right direction would be great. I'm most interested in finding more books to read (especially the cookbook-type as mentioned above), but anything would be awesome.
Although I am not the right person to answer your question but as I read your query just want to suggest you something.
If you are already done with the beginner's book and you think that you have a good hands on knowledge on the java then try to clear the sun's scjp certification. You can refer a number of book for this one of the best amongst all is Kaithy Sierra's book for SCJP 1.5.
It all depends on your interest. Like Chirag, I suggest you go for SCJP. Having appeared for the exam myself, I know it is tough and absolutely tests your basics. If you're not interested in formal certification, you can search on sourceforge for projects in java which need help.