Barbara Taylor McCafferty and Beverly Taylor Herald are twins who write about twins in their Nan and Bert Tatum series. I just finished book number 3 (1 and 2 were not available yet from the library) and thoroughly enjoyed it.
Nan and Bert are middle-aged twins who are also mirror twins (look-alike but are opposites). Which means they look enough alike to pretent they are the other twin when needed. Sometimes it gets them in a little bit of a fix.
The book is a quick fun read. I am now waiting to catch up with books 1 & 2.