I think that Kogler had some good advice there. You are not a complete beginner, so if you have the practice time I'd suggest splitting your time between a few key areas. You might want to work through the fundamentals course to make sure that there are no gaping holes in your technique/knowledge, while at the same time challenging yourself with a song that you'd like to learn or have a look at one of the style lessons.

Don't forget to enjoy the ride!

Edited to add an actual answer the original question- I think that with creative use of the resources at GT you should be able to make a practice schedule to improve considerably. You probably won't need any other books unless you progress to an advanced level and want to really specialize.