I got my first amateur radio license in 1992 – it was the no-code Technician class. I’d studied the Gordon West books found at Radio Shack back then. In fact my Tech and General I used those books.

For my advanced class license I used the ARRL Study books. The one I used took a deep dive into theory which I loved. Loved it until a buddy pointed out the pattern in phase angles.

For my Extra class license I drilled on the question pool. You can download them for free if you search on the web.