Can we please clarify the rules regarding the use of books in correspondence chess?

I frequently see the claim that the opening book in the analysis board is the only one allowed in correspondence games. My understanding is that this is not correct and using any book is allowed: Win with the London System, How to Reassess Your Chess, Dvoretsky's Endgame Manual, etc. Also, using YouTube videos would be equivalent to using a book, and therefore allowed. This is all correct, isn't it? I think the FAQ should be amended to include such examples, or at minimum, this should be confirmed in this thread by a mod.

Also, using an endgame tablebase is NOT allowed, as implied by the tablebase's absence from the analysis board. And, for the sake of completeness, using an engine and asking a friend for help are both already explicitly against the rules.

And... crickets. Look, don't you think it's important that everyone playing a game be in agreement over the game's rules? It's certainly not very fair if some players think they are allowed to use an external book and other players don't.

lichess.org/terms-of-service

1. Cheating. We define this as using any external assistance to strengthen your knowledge and, or, calculation ability to gain an unfair advantage over your opponent. Some examples would include computer engine assistance, opening books (except for correspondence games), endgame tablebases, and asking another player for help, although these aren’t the only things we would consider cheating.

So using an opening book in a correspondence game (ONLY) is permitted on Lichess, as it is on some other websites.

But using ANY assistance outside of an opening book during a correspondence game is against the ToS.

^ what keith said: (1) don't use computers or outside resources while you're playing and (2) you may use opening book provided by lichess

See? This is exactly the problem I'm talking about. So far there are four people in this thread and at least three different interpretations of the rules.

1. My interpretation was: Any and all books/videos/DVDs are allowed. This would be consistent with the way correspondence chess was played before computers came along and messed everything up.

2. adamimorris came in and contradicted my interpretation: You can use an external book, but it must not contain engine analysis, and it MUST be strictly an opening book; middlegame and endgame books aren't allowed unless the book contains the moves from the beginning all the way to the position in the book.

3. KeithDenning pointed to the TOS (which I've already seen; its vagueness is why this thread is necessary in the first place) and claimed that an "opening book" is allowed, but said nothing about what "opening book" means. This would *seem* to match adamimorris's interpretation (except possibly the bit about engine analysis, since the TOS says nothing about whether the opening book may contain any), except...

4. FlamoToolz, a lichess mod, stated agreement with KeithDenning, who appeared to agree with adamimorris, but then implied that you can't use ANY books AT ALL, except for the opening book provided by the analysis board—completely contradicting adamimorris!

This is insanity! We can't have meaningful competition if nobody agrees on what the rules are. The TOS as worded seems to me to heavily imply that, at the very least, using an external opening book is OK (since the terms mention "an opening book" and not "the opening book found in lichess's analysis board"), but FlamoToolz doesn't agree—which means either the wording of the rule is deceptive, or else even the mods can't get the rules straight. Both of those are very bad.

I feel like we aren't even getting to the distinction of what "opening book" means in this context
The implication I am getting is that you are strictly referring to something like a TCEC "opening book", just a set of random moves given

The way I (and basically anyone I've ever talked to) always understood the rules was that you are allowed to use A BOOK ABOUT THE OPENING, eg www.qualitychess.co.uk/products/1/290/playing_the_ragozin_by_richard_pert/
Naturally every single opening book (in the sense of the word, an opening treatise by an author) will have engine analysis (and evaluations) if it's written post-1960, so "You can use opening books, but only ones that don't use engine" is a bit of an oddball statement

If the official ruleset is indeed that you CANNOT use outside materials such as chessable courses, DVDs, quality chess GM Repertoires, etc, it DESPERATELY needs to get rewritten (and there's nobody out there who is following that ruleset)

E: The way City is phrasing it sounds again to me the way I understand it ("Opening books" are allowed, just no engines)

I had (and have) the same impression as you @furrykef. I don't see @KeithDenning explicitly saying 'lichess opening book' but rather says that an opening book is allowed on lichess (as other sites) ONLY for correspondence.
@adamimorris ' comment I don't understand. Is there an opening book not based on engine analysis today?
So the question to @FlamoToolz is do you mean 2) you may use the opening book provided by lichess and no other book?

@Zubenelgenubi as I mentioned, there is a 2ndary definition of "opening book" beyond "physical book with analysis of an opening": Effectively just an assortment of moves, usually without even an evaluation - either for computer chess, as a "they use these openings" (eg the 'TCEC opening book' will contain all the openings that the engines play against each other at TCEC), or as simply a different term for "database", so you would be allowed to use something like this database.chessbase.com/ as your 'opening book' (not this one specifically, as it displays engine evaluations also, but in general).

The point of contention here is whether the rule text claiming "You can use an opening book" means "You can use Quality Chess' newest GM Repertoire, and have Negi's top of the line analysis guide you through the first stage of the game", or whether it means "You are allowed to use a database (but only if it doesn't contain engine evaluations)".

Everyone I know assumes the former, but in that context "Not allowed to have computer analysis / evaluations tho" makes no sense (as all opening books contain those), and "an opening book" is also slightly strange wording, since you would be allowed to use multiple opening books (rather than "an").

The latter makes more sense 'in every way' (also eg to make correspondence not just a "who bought the best book" contest), but if the latter is the actual rule, it needs to be worded entirely differently, as nobody is gonna see "you are allowed to use an opening book" and read "you are allowed to use a database (as long as it doesn't contain engine evaluations); actual books are forbidden btw".