When it comes to solving puzzles, whether thematic or not, it seems to me that the only intrinsic resource is the solver's brain, supported by such references (if any) as are
explicitly recommended for that puzzle (whether in paper or electronic form), eg 'Chambers Dictionary (2016)'.
Beyond that, I cannot see that any material or medium has automatic precedence over another - the solver must decide what they think is most appropriate, whether that be thumbing through ODQ, typing a couple of words into a Google search, playing a CD, or, if the theme appears to relate to a board game, getting their Monopoly and Cluedo sets out of the loft. As far as I'm concerned, these methods all constitute 'external help' of one sort or another, whether the objective is to resolve a reference in a clue or to unravel the theme of a puzzle.