Especially considering that a correspondence game can be days or even weeks between moves when using actual post, and correspondence players have the habit of playing many games at the same time, its impractical to try to prevent high level chess players from looking at any book covering every opening or end game that they might be playing in.
Imagine the alternative, at what point during a correspondence game should it stop or start being acceptable to study a bishop and knight checkmate? At what point should it be acceptable again to pick up a book or watch a twitch channel after you've posted 1. e4 to someone? I'd need the book just to tell me that I was out of book if we still had even material, though I'd probably assume I wasn't in book any more if I was down a piece.
Worth sticking something in the blurb for future years?
Imagine the alternative, at what point during a correspondence game should it stop or start being acceptable to study a bishop and knight checkmate? At what point should it be acceptable again to pick up a book or watch a twitch channel after you've posted 1. e4 to someone? I'd need the book just to tell me that I was out of book if we still had even material, though I'd probably assume I wasn't in book any more if I was down a piece.
Worth sticking something in the blurb for future years?