I've just found the most enlightening book on parish registers,and everything that you would ever want to know about them is in it. I couldn't put it down till I'd finished all 130 pages,well worth downloading the PDF version as it is only 8.6MB. It answers every question I've ever seen asked on the forum and a lot that haven't been asked yet.
I found it after following the post by: royston on: 08-10-2008 "loads of free items for genealogy."

THE PARISH
REGISTER
BY WILLIAM BRADBROOK, M.R.C.S.
1910
CHAS. A. BERNAU, WALTON-ON-THAMES, ENGLAND

http://www.archive.org/details/parishregister00braduoft