Hi Serenity - While it's not in the DB/DR books (so far as I recall), I think those letters are also advocated here. They're sometimes referred to as letters of release - and I think a lot of us have written them - some sent, some just written and filed away. That letter can be a very powerful way to let yourself know where you are - and also a way of letting your S know that you are comfortable enough with yourself to let him do what he has to do...it's part of reaching a point of being loving though detached.
I've never read the book Love Must be Tough - so I can't really speak to that one, though.
I thought the Five Languages was pretty useful.
One of the best books I read is The Seven Principles for Making Marriage Work - by Gottman...which, ironically, I read after I had decided I no longer wanted to work on my marriage...