If you are a family history researcher this book will be invaluable. I was given it as a Christmas present, after dropping several heavy hints, and have already made good use of it.
What you get in the Atlas is a map for each county in England (and Wales and Scotland) showing the pre 1832 parishes, the earliest dates when parish registers were recorded in each parish, with eclesiastical jurisdictions and "peculiars" all colour coded. There is also a topographical map for each county, though I find I need a magnifying glass to make good use of these.
The Index in the second part of the book lists the parishes for each county and then gives the registration district for each parish. This is invaluable information when looking for an ancestor's birth, marriage or death in the post 1837 GRO registration indexes, and you can use the map references to see at a glance where each parish and registration district is.
So that is briefly what this book is all about, and if you are still reading this review you are probably a serious researcher and it will be very useful to you. Definitely not bedtime reading, but one to put on the shelf and consult again and again as a reference book.
I should add that this is the third edition of the Philimore Atlas, dated 2003, and a lot of the census information in the Index is no longer as valid, as all the censuses are now available on the internet, although for a price of course!