First of all, before doing anything, I would suggest to think about which GD&T standard is mandatory or more common in country/company where you work. Most frequently used are:
- American ASME Y14.5;
- group of ISO (European) standards.
There are some very fundamental differences between these two and IMHO this fact should have significant impact on whole learning process (choosing proper handbooks, training company etc.).
Being aware of this I would suggest taking really comprehensive live training on fundamentals of GD&T. You will not gain expert knowledge after completing such course but for sure it will help you in better understanding of any book you take later on. It would be also good if you have a copy of a standard you try to follow.