PIN POTHA (puñña potha) or Merit Book, costom still alive? How to keep it up?
As a Sri Lankan, I've heard these in our grand parent times (within last 20-30 years back). With the current lifestyle, everyone has started looking for happiness in outer world, most of the people have forgotten these kind of customs / traditions. This doesn't imply that Pin Potha is no longer active in Sri Lanka, but I should say its very hard to find now a days.

As I've heard the main purpose of this book is, when the owner of the book is about to die and while laying on the bed (Here it's talking about natural death with aging, because that's how the most of the people died in back old days, no sudden heart attacks) someone else starts reading the book to the owner. The concept behind this is, to make his next life a better one ( next birth in human world or heaven not the hell) 

How that happens is the bhava in Pratītyasamutpāda is caused by the previous Nidanas. So if he can have good Saṅkhāra (mental fabrication) based on what he hears (from Pin Potha) at the last fractions of seconds, it will results to create a good Bhava. This is the concept behind this action as I've understood.