How important is a fish finder?

[jonrhett]

I can't wait to start using my fish finder. I've found a lot of random reading material online about how to fish while using one.

But does anyone know of a good book or guide that would help me learn how to use a fish finder while fishing?

I know the user manual will help me with how to work the unit, but I guess I mean more like what to use on what type of structures etc.
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