This is not your average "which book to learn from" question. I'll post my research and background in as much detail as possible (since this could potentially help others with similar question).
I've been playing guitar for over 2 years and began piano at the end of last year.
I did go for lessons for the guitar at a local institute when I began. We used Mel Bay's Modern Method for Guitar Grade I. It was a 3 months course, and I accomplished more than what I could learning alone. I also subsequently did well on Trinity Grade I.
The way we worked through the pieces was to start practicing a piece, and demonstrate the piece to the teacher when you could play it fairly in time. It needn't be perfect. After completion of the Primer book my instructor told me to go back and try playing the starting pieces. I couldn't believe how easy they now were, even though I had never practiced them for a long time.
In short, going to an institute was a boon, since I progressed rapidly. However, it was not just about having an instructor. My institute had many teachers there, being available at different times. Also, my timings were flexible. A single instructor never taught me. So, an instructor knowing my strengths and weaknesses and guiding me through it is out of question. But still I improved.
Now lets get to self-learning. I've been pondering over Berklee's Modern Method for Guitar Vol 1 (for the last 1 year). The book mentions regular review, and to never get stuck on a single piece. But how do I know how much of a piece to practice initially? When do I move on to the next piece? When should I come back and review?
I find this question more important than what exact book to follow. Any experienced folks who can suggest something? In short, I'm asking a very theoretical question of "how to learn" and "how to maintain a learning path."