Very free form. Once you find a few interesting sources in the subject you're studying, you're in business. That's because a) you develop a huge reading list from the works cited in the current work, and b) you come across things you didn't know and then you have new avenues to explore. I think you need a split between theoretical and practical knowledge, although the exact split probably depends on the field. If all you possess is practical knowledge, then you're probably missing a lot about why it is you're doing what you're doing, and you're probably being terribly inefficient, as well. If all you possess is theoretical knowledge, then you're probably sitting in a chair doing nothing but thinking about how smart you are. The good part is that if you're truly interested in the material, even dry technical crap can be very captivating. Do what you're passionate about and do it all in.