Is it necessary?

[Ann]

Nope. I don't think so. I started out reading The Sword of Shannara rather than First King of Shannara. I didn't feel like I was in the dark about anything.

[Elentarri]

No. My first Terry Brooks novel was the Elfstones of Shannara. I read First King of Shannara several Brooks novels later. It's a stand-alone novel filling in a gap in the history.

[Edward]

I would actually recommend AGAINST reading it first. It spoils some plot points for later in the series and has some easter eggs that make no sense if you haven't read the others.