Approach for doing Entity Relationships I'm looking for the best way to approach creating relationships using Drupal 7.  I am looking for advice on creating the following:

1) A "letter" content type (as in an old fashioned letter)
  a) From Person (i.e. "John Doe")
  b) Associated From address ("12348 Street")
  c) To Person
  d) Associated To address
  e) Postage Date
  f) File attachment of scanned letter

2) A person can have one or more addresses (I am recording some family history of letters sent and people have changed addresses over time)

3) Ability to select the Person/Address in the Letter content type for the From / To Person.  

I've been reading the documentation, but I'm just unclear on the best approach.  Would appreciate suggestions. For example, I started out creating a field for From Name and To Name and From Address and To Address, but realized I'd be creating a lot of redundant data and that it would be better to create relationships, but am not sure how to accomplish that.  Should I create a Person Content type?  If not explicit instructions, would appreciate a general point in the right direction.
Just general ideas, not a solution...

I would check out the entity reference module (https://drupal.org/project/entityreference) as a starting point, to get an idea of how it works, so you can plan your project with this in mind. Entity reference makes it easy to link one entity to another, then build views to generate lists of related items. Entity views attachment (EVA, https://drupal.org/project/eva) can help you attach views of related items to entities. (i.e. you could display a list of letters as a view attached to a person)

You may also want to check out the addressfield module (https://drupal.org/project/addressfield) especially if you have international addresses to work with. It's a little tweaky, since it handles the entire address as a single field, but it saves you the trouble of implementing several fields to catpture each address. 

From your description, it seems like you would want at least these content types: letter, person, address (or place). But it's hard to say for sure without knowing your complete vision.