Most preamps are pretty basic units. They raise the signal to line level. Gain controls the gain, might have a low cut filter to cut lows, or a few extra features that you can mess with.
There's not really anything you have to 'learn' how to use. What's important is to know how it sounds so you can make a better judgment on what you need in a preamp, if anything.
Most cleaner pres that are decent and not noisy (DMP3 for example) don't really add anything to the sound. So if the source sounds good then you should be okay. If you want something 'warmer' or 'smoother' sounding then you would probably want to get a *nice* tube pre. These aren't cheap. I think the preamp is one of the best places in the signal chain to add character to the sound. A $50 tube pre is not likely to do this well, if at all.
Preamps aren't really like most other gear in the sense that "I need to learn this unit inside and out before I can be worthy of a more expensive one". Each one is different and it's the character that you need to learn.
And also what gain settings sound best on certain instruments and mics. For example, a little over 0dbVU might sound okay on electric guitar (or not), but bad on vocals. A little less than 0dbVU might sound a little cleaner on acoustic guitar, etc. These small subtleties are unlikely to be heard well by someone inexperienced or on most clean pres and are dependent not only on the preamp but the other gear in the signal chain.
What I'm saying is these kinds of things usually have to be relearned each time you get a new pre because each one is different. So don't hesitate buying one because you think you don't know your gear well enough.
You may get more familiar with pres in general and how they effect the sound, but you will still have to take the time to learn your new pre just like the one before.