Learning the basics:

Intro to bipolar: The National Institute of Mental Health offers a very informative introductory booklet. 
Statistics: Data and infographics from the National Alliance on Mental Illness. 
Might I have bipolar?: This self-test offered by Mental Health America does not provide an official diagnosis, but can give you a better idea of where you stand. It would be helpful for any psychiatrist that you visit to see the results, also.  

Reading about others’ experiences:

BPHope: This magazine-based community is by, for and about people with bipolar.   
Depression and Bipolar Support Alliance: Their section called Life, Unlimited is a collection of success stories (big and small) written by people with bipolar.

Connecting with support groups:

National Alliance on Mental Illness
Depression and Bipolar Support Alliance
International Bipolar Foundation

Resources for bipolar allies and caregivers

Living with Someone with Bipolar Disorder 
What Does Bipolar Feel Like?