I started one when I was a kid - maybe 14. Nothing quiet like growing up poor to try and motivate you to get some $$$.
I took an area inwhich I had a lot of knowledge (trumpet playing). I started collecting anecdotal types of information from pros and stuff I head leared and compiled them into a booklet called "Top Trumpet Tips."
I then sold that booklet in the classified sections of publications. The first one I tried was maybe not the best fit - Popular Mechanics. But I sold quiet a few. Probably not enough to break even.
Then I bought ads in the musician and trumpet guild publications. They turned out pretty well, but I don't remember if those broke even or not, but they did sell better.
Every week I would walk a few blocks to the local drug store/post office, make copies of my 'booklet' and mail them out. I think I sold them for $6.95 + $2.00 S&H. This was 12 years ago, maybe more.
I did one or two other projects like this, and tried one or two other things that didn't get off the ground.
And guess what? I'm at it again.