Here are a few links that were on my old blog, covering what agents (can) do for you, what you should know, and how you can work together. Some of these will, I hope, lead you to other resources, too! These are older posts, but the information is timeless.
AGENTS and INFORMATION
Rachelle Gardner on letting your agent run interference for you and what to ask an agent if you’re offered representation
Terribleminds.com on 25 things writers should know about agents
Jane Friedman on how to tell if your agent is any good
Nathan Bransford on what literary agents do
Robin Mizell on agents who only consider published authors