The definitive book for anyone who is interested in the history, geology and symbolism of the slate headstones found in many of the churchyards in Leicester, Leicestershire and some of the adjacent counties is David Lea’s “Swithland Slate Headstones“, which includes headstones in St. Peter’s churchyard. Not only is it a comprehensive source of information, illustrated with high quality images, it is also available as a free download from the “Heart of Albion Press” at the link above.
The PDF can be downloaded for free, but please note it’s over 80Mbytes so maybe slow on less-than-ideal web connections.
An article on Charnwood slate gravestones in Northamptonshire and Lincolnshire (which suggests these are evidence of ‘return loads’ along trade routes) is available on the same webpage.
Also available on the same website is Alex Hando’s extended essay “Cleft from the Hills” which explains why the slate industry of Blaenau Ffestiniog was more successful than the Charnwood Forest quarries.