If you have fifteen minutes to kill, you might not have time for a game, but that’s plenty of time to do some Go problems. First of all there’s Goproblems.com. In my opinion, the best problems there are in the time trials on the right. The Senseis’ Library wiki has a section of beginner exercises. Gobase has a set of Korean academy problems, as well as facilities for replaying professional games, but they’ve started requesting a donation for joining.
There are a lot of really good go books out there. Tesuji and Life and Death by James Davies are both extremely good. Tesuji is easier, while Life and Death has a very broad range of problems. If you had to pick one book that’s helped me more than all others, it would be Attack and Defense from the same series (Amazon doesn’t have it, but other places do). Kageyama’s Lessons in the Fundamentals of Go is one of the most popular books out there. It’s got a really nice conversational feel, and helps with big picture aspects of your game. I also like Fundamental Principles of Go by Yilun Yang, though it’s probably for somewhat more advanced players. Lastly, David Carlton has an incredibly in depth set of reviews for go books published in English.
Once you’ve been playing games for awhile, it helps to start reviewing them. That’s easy if you play on KGS, since it creates a record of the game. Otherwise, just sit with your opponent and try and replay as much of the game as you can. The more that you try this, the more you can remember. If you can, get a stronger player to have a look at the game and advise you. There are a lot of people who might be willing to review your games, including several people in the Pittsburgh room on KGS. Otherwise, just exchange ideas with your opponent. When you get to a move that gave you a hard time, try out different variations, or try to identify mistakes you made. You can also submit games to the Go Teaching Ladder.