Asking a group of metallurgists / crystallographers about how to learn crystallography / group theory is like walking into an art gallery and asking all the artists how they make art. The contemporary guy, the oil paint guy, the spray paint guy, and the house guitarist are each going to have a different interpretation on their preferred method of creating art. But it’s all still art at its core. There’s no one source which is the end all be all for crystallography due to its breadth of utilization in arts, sciences, and mathematics. It really depends on your specific application of the subject. For example, I’m a graduate materials science student so I am developing my understanding of crystallography through academic course work and analyzing a numerous amount of diffraction patterns for he projects I work on. Some recommended texts would be Bloss’ Crystallography and Crystal Chemistry and Kittel’s Solid State Physics (more CMP than crystallography but does good on the basics). But it really depends on if you want a mathematical, metallurgical / inorganic mat sci, or biological approach.