Particularly in the recent Brexit threads, there’s been a lot of questions about what is actually pretty arcane Parliamentary protocol and procedure.
You’d think it would be quite easy to answer questions about that sort of thing in a modern democracy, but no! This is Britain we’re talking about, after all. 
The key document on how it all works is a treatise that was first published in 1844 by Thomas Erskine May, who was Clerk of the House of Commons at the time. It was originally titled A Treatise upon the Law, Privileges, Proceedings and Usage of Parliament, but it’s now just called *Erskine May: Parliamentary Practice
*. It has been continually updated and it’s the document the Speaker and the Clerks use to figure out what is allowable etc
Up until today it has only been available on paper, and damned expensive paper at that - the 24th edition can be had from Amazon for as little as £300! I doubt any Doper has a copy at home.
The 25th edition, published today, is online and free: