The following answer was deleted by a mod despite containing a citation of an in-print textbook Notation for two entangled registers
Is it really the purpose of moderators to clamp down on potentially useful information?
I believe the Moderator acted correctly. The original answer was written with little more than:
"A more general answer to the question … can be found in Chapter 10 of [link to book]"
Folks here will continue to work hard to build this site, so when someone finally finds this community through search, the last thing we want to do is send them elsewhere to find that information.
I understand your concern why someone would remove a link which would seem to satisfy the imperative to answer a question, but Stack Exchange was not built to be a search engine nor a list of links. The purpose of this site is curate an first-hand documentation of knowledge by authoring answers which add value to this site specifically — answers which can be vetted by the community with a wiki-style editing which allow us to edit and improve that content. You can't do that with a link.
Folks often describe how links can break etc, but I think that entirely misses the point of why links are not considered an "answer" in the context of this site.
I'll be adding a bit to @Robert's answer.
This was your original answer (v1):
A more general answer to the question, why can't the state of the extra qubit be written in the form α|0⟩+β|1⟩ can be found in Chapter 10 of Rieffel & Polack, Quantum Computing - a gentle introduction (link to pdf).
And note that I had added this comment while deleting the answer:
Please do not post links without at least a basic summary of what they contain and how that content answers the question, since link-only answers become useless if the link rots away. Link-only answers are not considered answers here and will be deleted. I've converted it into a comment for the time being.
Your original "answer" is still available as a comment on the main question, so I don't think any useful information has been clamped down. In the future, if you get time and manage to edit and elaborate on the answer, according to the guidance given, consider flagging it for undeletion (click on "flag" → "in need of moderator intervention"). Meanwhile, if you have any minor additions to make or clarifications to ask for, use the comment section.