Hi, what do you mean with booking? 

If you have an Eurail Pass, this is your ticket (you need to add the trains in the app or write them on the paper version) and you are good to go.

Seat Reservations:

some trains can't be reserved, only hop on the train (like Trenord local trains - malpensa Express to Milan Malpensa Airport not included in the pass)

some trains have optional reservation, mainly long distance trains in Germany, Austria, Switzerland or Czechia.

some trains have mandatory reservations like long distance trains in Italy, France or Spain.