I recently received a “how-to” email from a friend about how to email. It gave good advise about how not to make the mistakes that lots of people make, for instance replying “okay” without including what they are replying to. I like good advise, but ignore the parts I consider bad advise.
“The Effective Emailer” also gives mostly good advise. A bit of it is business oriented (“to help you become a more effective emailer”), but that doesn’t mean it doesn’t also apply to personal emails.
Ignore the parts that don’t work for you. Rules are made to be broken, but first you have to know the rules.