Before you start researching exhaustively, realize that there are a couple of different types of efficiency that are involved in brewing.
Mash efficiency is a measure of the percentage of the total possible sugars that are extracted from a grain bill.
Brew house efficiency is a measure of the percentage of the total sugars that actually make it into the fermenter. This includes losses to a variety of places in your system like dead space in the mash tun and boil kettle.
Proceed with your research.