Published books about United (3)
I believe the answer is:
out
'published' is the definition.
(I've seen this in another clue)
'books about united' is the wordplay.
'books' becomes 'ot' (Old Testament).
'about' means one lot of letters goes inside another.
'united' becomes 'U' (abbreviation).
'ot' going around 'u' is 'OUT'.
(Other definitions for out that I've seen before include "Not allowed - on strike" , "Revealed; unconscious" , "Walking back to the pavilion" , "Not indoors" , "Inaccurate - dismissed" .)