There is a lot of theory information out there and you can become paralyzed with theory (Have been there). I agree with BayouBill above, keep it simple. I would look at some Jazz Fake Books, good chord progressions and harmony in them. Check out some short and simple theory specific YouTube Videos, ex. like when to use a diminished scale for example. These short and simple methods help you to retain the information and really understand it.
In your example above, if you know 2-5-1, you already know what Tritone substitution is, indirectly. Take for example Em7 A7 DMaj7. A Tritone substitution in this case would replace the A7 with its' b5, or Eb. So your progression would be Em7 Eb7/Eb9 to DMaj7. Note that Eb is the tritone sub for A, and, A is the tritone sub for Eb. They are interchangeable. Play a Bm pentatonic over Em9, play an A arpeggio over Eb9, and then a Bm pentatonic over the Dmaj7 and you will hear the effect of the tritone substitution and likely remember it a long time. Or you can do the opposite, if your progression remains Em7 A7 Dmaj7 try playing an Eb major (Cm) Pentatonic over your A7 chord. You will catch a lot of "color notes" in there in either scenario. If you are a guitar player you will start to see these patterns on the neck and store them in your toolkit for future use.
Regards