I was pleasantly surprised to see all my favorite O'Reilly books, but also amazed at how much new technology has been developed over the last few years. Areas that I thought I was very familiar with have suddenly shifted to include new tools, and new ways of doing things with cloud, big data, and all kinds of new Apache products.
Check to see if your company provides free subscriptions, and then have a look at some of these resources:
http://www.safaribooksonline.com -- Huge selection of technical books
http://www.pluralsight.com -- High Quality, from overview to advanced topics for programmers
http://www.lynda.com -- High Quality, some topics are not very advanced, but courses are very well done
When discussing career development ideas with my colleagues, I usually recommend training courses, or books, or documents to improve skills in some area.
One reason a formal class is so effective is that the homework forces the student to learn. I had a book with the title "Data Structures and Algorithms in C", which I had every intention of digging into "some day." The examples are difficult to understand, and code snippets seemed incomplete, so I felt kind of lost. Then I took a "Data Structures and Algorithms" class at UCSD Extension, which used that exact book. Sure the examples were still hard to follow, but taking a formal structured approach to study made the book come to life. Of course, a good instructor is an important part of any class, but the point is, that having a reason to dig into the content every week made the learning experience much more valuable.