When I decided to return to college after a twenty-year hiatus, where did I start? At the local community college! I spent a year there taking courses in history, geography, as well as differential and integral calculus. That was excellent preparation for what awaited me at the University of Colorado. I am very glad that I did so. My calculus instructor was the equal of any mathematics professor I studied under at the university. In some respects she was superior to many of them for not only did she know the subject matter, she was an outstanding teacher as well. The same cannot be said of many college professors. They know the subject, but their teaching abilities leave much to be desired.
I have a few book suggestions for you, JonathanH, that I think you will like. Any of the "...For Dummies" books are very good. I have one on differential equations that I particularly like. Lots of worked out examples as well as easy to read, but very informative text make it a keeper. They publish many titles in mathematics. Also, another series I like is the "...demystified" books. The ones I have on linear algebra and complex variables are wonderful. Both series have plenty of mathematics titles. And don't forget the Schaum's Outline books too. All of these series are easily available online at outlets like Amazon.com.
I've heard good things about the Khan Academy online educational system. I'm glad you are taking advantage of it. A friend of mine has decided that he would like to study mathematics on his own. He has never gone to college and is now in his early sixties, but what he saw on the Khan Academy website really piqued his interest. So he looked at the lectures on trigonometry and now wants to learn more. Online education is going to have significant impact and I am so happy that it is available to anyone with a computer and Internet connection. Maybe some Witnesses will shake off their intellectual torpor and finally pursue worthwhile knowledge.
One last thing. Don't give up on your writing dreams. I am a published author myself, and am currently working on a fantasy novel that has the mathematics of linear algebra as a key element of the story. And if you're looking for some good fiction that features mathematics, let me suggest Arthur C. Clarke's The Last Theorem, Isaac Asimov's Foundation Trilogy, and Mathenauts: Tales of Mathematical Wonder, edited by Rudy Rucker. It includes Larry Niven's delicious tale "Convergent Series".