I think studying GM games helps me a lot actually.
Anyone that tries to learn a new opening, especially one that next to nobody plays (in my case 1.b4 - amazing opening btw!), will see that it is almost impossible without looking into professional games.
Yes, often you can be learning by doing, but this means often that you will unknowingly train inaccurate or even wrong moves.
If you look through GM games, the best way of learning is to learn every move by hearth and being able to repeat them in parrot fashion. It is not necessairy to understand every move perfectly, often one must accept that this certain move IS the best. In some books it is not told, why those moves are the best, but in the end it comes down to certain variations that could happen, which are in comparison worse.


Oh and I read "My System", which I rated pretty highly. But I wasn't classing that among the "old books" because that was the begining of the counter-reaction.