I am rather new to note investing. I recently bought and attended a three day course on non performing notes. There is currently opportunity in notes due to the glut of toxic assets from 2008, which are finally making their way into the market. These can be highly profitable. Once that is over, it will be time to reinvent. The attraction of non performing notes is that they require less capital and can yield high returns.
Less dependent on bad notes is the performing notes side of the business. These are less risky and provide a steady stream of income. Performing notes require more capital and yield decent returns.
You need to know what kind of note investing you want to do.
I would suggest some of the top resources, Noteschool with Eddie Speed, We Close Notes by Scott Carson, PPR with Dave Van Horn, and Note Queen with Dawn Rickabaugh. Carson and Van Horn hang out here on BP, if you want to look them up. From there, you can find resources for buying, selling, and servicing notes.
Good luck.