in my experience a gd teacher is invaluable, reading a book can be really boring sometimes but having a good teacher can almost spur you on to keep going..... trust me, it will take YEARS to learn even half the stuff (i know as ive been doing it for like 5 years or so)
There's an article on here called The Crusade. Its pretty good, Id check that out before you payed for anything.
search the internet, and learn what you can first. Then remember what it is that you have trouble with (circle of fifths, Notes in a key, whatever) and get a teacher AND a book. I had a guy teach me for a lil bit, and since I had done some background study, I didnt need to go through everything that I knew, I just proved to him that I new it well enough and we skipped it and went onto stuff I didnt know as well. So just try to learn off the net, and when you have problems with something just write down on a piece of paper what it is and then look to cover all those things
you can teach yourself theory if you have a pretty lengthy attention span and a really good source of information but personally i think a teacher is the best way. They can help you understand how you can apply certain aspects of theory to your playing that most people would miss. I tried to teach myself theory and didnt get much more than a good understanding of the pentatonic scale out of it. In two months of having a guitar teacher i have more than doubled my understanding theory and looked into stuff that i thought i know before.