King George V Three Halfpence Die 1, Flaw Location Guide, Electros 1-12 and Electros 13-29 by Bill Fiora. (2nd editions)
These Guides (2 volumes) provide a comprehensive list of, and drawings for, all known flawed cliches for the 11/2d Die I stamps, many of which have not been previously published.
The Guides are extremely user friendly, and have a complete cross referencing system, which allows simple identification of any stamp that has a constant flaw or flaws.
If you collect these stamps you need these books !