Objects in space are specified by their Right Ascension, Declination, and distance. The first two are easily measured, usually to better than a part in a million; the last is notoriously tricky to measure, sometimes uncertain to an order of magnitude.
The time derivatives of these quantities are the reverse: proper motions are unmeasured for most objects in the universe, but velocities can usually be measured to a part in a million rather easily.
I noticed this (I’m sure I’m not the first) when writing a review chapter on precise radial velocities as an exoplanet discovery method. I think it’s a good primer on the subject for students just getting started. In it I briefly trace the origins of the method to the fundamental importance of radial velocities to astronomy in general and spectroscopic binary star work, then work through the high-mass-ratio limit of SB1s, the first exoplanet discoveries, and the future of the method.
There is also a quick section giving what I think is a fair overview of the problem of stellar RV jitter, including the roles of surface gravity, granulation, oscillations, and magnetic cycles.
You can find it here. Enjoy!