How to find out if a Document is part of a book?

tell application id "com.adobe.indesign"

  -- exists (book contents of every book whose full name is full name of active document)

  full name of active document is in (full name of every book content of every book)

end tell

If the search should only consider the opened books, the commented out line would be the fastest (in execution time) solution. Unfortunately Adobe never supported the full power of AppleScript, where you could push that kind of expressions into the scripted application. The line below works by building a list and performing the search in the AppleScript runtime.

Translating that to ExtendScript requires a shim for Array.indexOf ...

("…"+app.books.everyItem().bookContents.everyItem().fullName.join("…")+"…").indexOf("…"+app.activeDocument.fullName+"…")>=0

Btw, I would not rely on those strings from book contents without thorough testing - books can get moved in the FS. Some of the properties used to be redundant and/or only partially maintained when a book recovered from such a move, I never checked whether that was cleaned up internally in the file format when the bug was fixed ages ago.

Dirk