I (have*) spent two days reading all the letters that I have received throughout the years from your parents and you since 1974.
have is optional:
* you must use it if what you mean is that you have been receiving letters until 'now' or a point in time very close to the present.
* if you don't use it, the meaning of the sentence would be that you read that letters and that you haven't received any others since then.
Student or Learner