Erskine May is the authoritative book on parliamentary law and practice. It’s a description of how procedure in the House of Commons and House of Lords has evolved and the conventions that apply, rather than a set of rules.
The latest edition of Erskine May, published in 2019, is publicly available on Parliament’s website and will be updated periodically. You can also buy a hard copy or borrow one from the House of Commons Library.
The first edition was published in the mid-nineteenth century and new editions are published approximately every six or seven years.
Its full title is A treatise on the law, privileges, proceedings and usage of Parliament but it’s referred to as Erskine May after its original author Thomas Erskine May.