In a debate, a bookstop (a.k.a. "required reading", "required viewing") is a requirement issued by one debater to another that they must read (or watch or listen to) some external matter before being able to understand an argument.
This requirement may be either explicit (e.g. "you'll understand after you read/watch/hear this") or implicit (e.g. links placed in a comment without any explanation).
An escalated form of bookstop in which one debater insists that debate is impossible due to the other debater's ignorance (and that the other debater must therefore read one or more works – typically unspecified – on the subject before meaningful debate can resume, while adamantly refusing to attempt to summarize the relevant information contained therein) is a form of argument from authority – in effect: "until you have studied what I consider to be essential works on this subject, you must accept my superior knowledge".
A more appropriate response in a rational debate, when one feels that another party is lacking in necessary background knowledge, is to summarize the necessary information. This may be prefaced or coupled with inquiries about the other party's knowledge of various elements of said knowledge, in order to avoid the need to explain those pieces which the other party does in fact already know.
Since sources and references may be useful, if a large work is referenced, preferred practice is to call out the specific elements which are relevant (e.g., by page or section identifier(s)), and if possible quote those briefly in the text of an argument.
When formally debating on Issuepedia, bookstops are forbidden by the clarity rule: it is the speaker's responsibility to clarify their argument, not the responder's responsibility to understand it.
To the best of my recollection, the term originated with the LessWrong community, but I cannot find any references to it on that site. Possibly I came across it in a dialogue in the #LessWrong channel on IRC, so it may have originated somewhere else entirely. --Woozle (talk) 12:07, 11 March 2013 (EDT)