When I was at school, we were told to start letters with Cher Monsieur if we didn't know the gender of the person we were writing to. At the time, I believe the rule was to assume masculine until proven otherwise.
This behaviour mimics the fact that a mixed plural is always masculine too (Paul et Marie sont grands whereas Paul est grand and Marie est grande). The saying goes le masculin l'emporte sur le féminin (masculine wins over feminine).
Similarly, it is usual to refer to the reader as le lecteur ; for example le lecteur intéressé se réfèrera au livre de Jules Dubonnet (The interested reader should consult the book by Jules Dubonnet).
However, there are people who believe this is a way of asserting a male superiority and that it's unfair towards women. The saying le masculin l'emporte sur le féminin in itself it particularly eloquent and can be understood as "men win" if taken out of a pure grammatical context (which it shouldn't be).
As such, if you want to keep a neutral stance, it may be better to either mention both (Chère madame, cher monsieur) or maybe use a plural (Les lecteurs intéressés se réfèreront au livre de Jules Dubonnet) which, even if it's still masculine, tacitly encompasses female readers too.