Erskine May's treatise on the law, privileges, proceedings and usage of Parliament (25th edition, 2019) is now freely available online.
The full text is available to search and browse via the parliament.uk website.
Erskine May details parliamentary procedure and is regularly referenced during debates. First produced in 1844 Erskine May continues to be regarded as the most authoritative work on the procedure and conventions for the House of Lords and House of Commons and attracts wide public interest.