When reading a design pattern book, the thing to keep in mind is that you are learning about design, and you want to know the pros and cons about each of the patterns, and when a particular pattern might be best suited to solve your problem.
I have read both the books by Mark Grand and Eric Gamma et la. The Eric Gamma did a better better job in explaining the pros and cons to me. If you have to buy just one design pattern book, I would get the book by Eric Gamma et la. Also go to amazon and read the reviews of what other people have said about these two books.
Note that the GoF book has sample code in smalltalk and c++, but that should not discourage you from buying that book if you only know java since it does a better job in explaining the concepts. If you want to see sample code in java, there are lots of resources on the internet, or pick up the Mark Grand book for the java code.
This site has class diagram and example java code (good if reading GoF book and want some java examples)
http://www.eli.sdsu.edu/courses/spring98/cs635/notes/index.html "The Design Patterns Java Companion" by James Cooper:
http://www.patterndepot.com/put/8/JavaPatterns.htm PDF book, and also java sample code
Bruce Eckel of Thinking in Java fame is also working on a Thinking in Patterns book. I haven't read his books, but there is a work in progress copy of the book at his website.
http://www.mindview.net/Books/TIPatterns/ Eddy
