Now, these do not include resources for How to Write a Query Letter or anything about what to send when you contact literary agents. This post only covers how to pick the best agents for you and your book.
Absolute Write Forums - http://absolutewrite.com/forums/
These forums are really awesome for the querying writer. If you have any doubts, questions, or concerns about an agency or even a specific agent, the forums can help you out. Use this to help you to determine if an agency looks like bad-news-bears or if a specific agent has a "no reply means no" policy. Knowing these things ahead of time can help you plan your list of agents to query and be confident and knowledgeable about what to expect after querying them.
Query Tracker - http://www.querytracker.com/
This source really helped me gather my initial list of agents. There are a lot of things you can do with the free account version, but I went ahead and upgraded to premium because IT'S ONLY $25 FOR A YEAR. The expense is minimal and I think it will really help me track my queries in the future. You can easily sort by your genre and see what agents represent that particular genre. You can also see comments from other querying writers about the specific agent's response times and request rates. These comments sometimes helped me pick one agent over the other at an agency.
Agent Query - http://www.agentquery.com/
This site was a secondary source to Query Tracker. Sometimes, Query Tracker didn't have that much specific information on an agent when Agent Query did. I went back and forth between these two sites to get the best idea on if an agent would be right for me or not.
Writer Beware - http://www.sfwa.org/other-resources/for-authors/writer-beware/
Writer Beware is great for alerts and notifications on publishers, agencies, and agents that have thrown up multiple red flags. Be sure to scan through their lists and updates and compare it to your list of potential agents to query. If any of them are on the Beware notices, you should do just that--be wary.
Publisher’s Marketplace - http://www.publishersmarketplace.com/
Although I haven't caved and purchased a subscription here yet, there is a lot of useful data to be had with a Publisher's Marketplace account. Maybe an agent's bio and interests sound like a perfect match for you, but do you know if they are actually good at being an agent? What if they have been active for two years but still haven't made a single sale? That doesn't sound promising for your career, even if they do end up loving your book and want to represent you. Publisher's Marketplace will give you access to agent data like how many sales they made, what genres they were for, and how big the deal was. This can help you identify the agents that could connect with your work AND are capable of selling it.
Literary agent interviews - http://www.writersdigest.com/editor-blogs/guide-to-literary-agents/agent-advice-agent-interviews and http://www.literaryrambles.com/
When looking for a little bit more information on agents besides their standard bios, interviews can be great. If an agent is going to be the person you select to be your business partner and greatest advocate, it's nice to know a little bit more about them besides where they went to school and a list of their career milestones. I found these two sites to be the most popular when looking for agent interviews. If you don't find the agent you are looking for in either of these two sources, I suggest Googling the agent's name along with the word "interview".
“by Agent Name at” Google search: I saw this tip on a post in an Absolute Write forum and it did make a lot of sense! If you don't have the money to fork over for a Publisher's Marketplace account, you can still get an idea of what the agent in question might (or might not) be selling. One way to check is to search--in quotes--"by AGENT NAME at". This is the terminology Publisher's Marketplace uses when writing up the deals it reports on. It would appear something like GREATEST BOOK EVER sold by Sally A. Agent at ABC Agency in the write-up. By searching in Google for the term "by AGENT NAME at" you should pick up that Publisher's Marketplace announcements. You won't be able to read the whole deal announcement, but it will give you verification that the agent has sold some projects and approximately when. Keep in mind, though, not all deals are announced via Publisher's Marketplace.
Agency websites have the final word: There are many resources out there to help us identify what an agent is looking for and how to format your query, along with what other samples to send along with it. Using other sources like A Guide to Literary Agents or Query Tracker are fine to create your initial list and set of requirements to query. However, I highly recommend scoping out the agencies website before you query, and use that for your final confirmation on what to send to an agent. Other sites and books can quickly become outdated, whereas an agency's website is usually the most often updated place to look. At least take a peek at their submission guidelines on their own website before hitting the SEND button.
So that's what I've learned so far in my querying quest. What about you guys -- are there querying resources that you live by? Help me expand my list and better prepare for the query trenches myself!