Erskine May
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Thomas Erskine May's guide to parliamentary practice is properly entitled 'A treatise on the law, privileges, proceedings and usage of Parliament' but it is commonly referred to as Erskine May (or simply 'May'). It is generally held to be the most authoritative reference book on parliamentary procedure.
First published in 1844, when Thomas Erskine May was Assistant Librarian, it is now in its 25th edition.