So my answer is, use a search engine like Google...properly used, it is the
best instruction tool available.

73



I am not EE trained and the ARRL Extra license manual worked very well
for me. A google search may work but, typically, you can get far afield
from what you want or need to pass the actual test.

My recommendation is to use ARRL material: it reviews the pertinent
material to allow you to pass the test AND it teaches you the theory
behind the questions. Not a deep treatise, mind you, but a good general
understanding.

My two cents.

John
AB8O