Improving My Personal Finance Management: Learning about income-producing investments

NCN of No Credit Needed has asked readers, What One Area Of Your Personal Finance Management Would You Like To Improve?. My answer came immediately: investments.

I think we have a solid grasp on budgeting, spending, saving, and paying off debt. The one place where we could use a lot of improvement is investments. Mostly short and mid-term investments, or as I sometimes like to think of them, income-producing investments. Because we're doing fine with retirement and even college savings, where the timeline is over 15 years (goodness, it is possible that Alex will be heading off to college in "only" 15 years?!).

So what I'd really like is an alternative income stream, something unrelated to our jobs that produces a fair amount of money each month. But my investing knowledge is mostly about short-term, liquid investments where the focus is on preserving capital (like CDs) and long-term investments (like index funds).

I'm lacking knowledge in the intermediate range, however. I don't think it should be too hard to learn about mutual funds that focus on generating income as well as growth, since I already know a fair amount about how mutual funds work. But I'd like to learn more about tax-exempt funds, as well as other instruments (like Treasury securities and TIPS). My learning process will probably be slow, just because of my schedule, but I'll post updates when there's something to share.