I've been subscribed to the Principles free weekly newsletter for about a year, and I believe Esrona is right. If you are really pretty new to playing/learning, it would be a good way to build essential skills while avoiding a lot of the mistakes and bad habits that are so easy to acquire. Even though I'm kind of an early-intermediate player, I still might get the Principles book.
For a structured, each-lesson-builds-on-the-previous-lesson approach, which actually teaches you how to play blues and has a real song to learn in each lesson, I would suggest you check out "Blues You Can Use" by John Ganapes. It's the best course I've found so far.
I tried the scatter-shot approach to improving technical playing ability, learning some theory, learning some riffs, etc., but find that for me it would have been a better use of my time to have a more structured approach. BYCU was the best $15 I've spent so far.
For reference, I took lessons for about 2 years back in the stone age, using the Mel Bay books. Learned some sight reading up to about the fourth position, but not how to PLAY GUITAR. Determined to learn to play about a year ago after a loooong time of not trying at all, and I've learned more in the first 6 lessons of BYCU than in my entire past up to when I bought it. Plus, I can actually play a few blues-sounding songs now.
When I think I'm ready for it, I'm going to get Kirk's
PlaneTalk course... just not there yet.
