Having a relationship with an agent / agency is a good thing even though they tend to take percentages for hourly rates or much larger chunks from the hiring company on your employment.
For contracting they offer a buffer of protection between you and the client aside from actually finding work. For permanent staff they actively hunt around to match companies and potential staff which takes a lot of pressure off you.
I’m not sure about the arrangements where you’re based but certainly in the UK and Ireland (and in most of Europe for that matter) it does not harm to ask around for recommendations and set yourself up with a LinkedIn profile so that they can find you too.
For all its ills, and however much I run it down for turning into ‘Facebook for Business’, it’s the only thing on the table right now as it’s so large.
For salary ranges there are a number of regular industry surveys you could check up on and many of the larger agencies actually publish their own.
Also, as I mentioned elsewhere, always push for the agency / client to give a range up front before you disappear down the interview pipeline!