Ladies of Letters was turned into a TV series a few years ago starring Anne Reid as Vera and Maureen Lipman as Irene, and it was the TV series that introduced me to the wacky world of 60-something-year-old pen pals.
Irene Spencer met Vera Small at the wedding of Irene's daughter, and it is a thank you note to Vera that starts off the friendship between them and the many misadventures they get into.
The first book in the series is hilarious. It sets out the main themes that will continue throughout the correspondence between the two ladies - from Irene denying Vera's digs at her gin intake to Vera's outrage over Irene getting involved in Vera's family affairs, to both ladies ending up in prison, or setting off on a fateful road trip.
Besides the TV adaptation, there has also been a radio production of the book starring Particia Routledge and Prunella Scales. I believe it was the radio programme that gave rise to the continuation of the Ladies' adventures beyond the original book.
I am really not sure which one of the pairings I prefer, but both somewhat remind me of a mixture between Golden Girls and Monty Python's Hell's Grannies.