I have always had a hand in hand play-a-long with theory, but I have played some good club players who never looked at a book.
They quickly learned to get out pieces and get stuck in maybe from other more bookish types.
I would be surprized if once you get to 1300-1500 the book makes a lot of difference. Its similar to learning openings by heart it has little effect on playing strength.
How much of a jump in play can one generally expect when one has never studied chess, and then studied seriously? Has anyone out there been in this situation that they have been playing online chess for years but never studied theory, then decided to study seriously? What were your gains? How did you start? Did your years of fly by the cuff play hurt your book theory?