I thought this use of coating was rather common, but looking in a selection of online dictionaries, I have only been able to find it documented in Collins, who define it (sense 3) as:
(English Midlands, dialect) a severe rebuke; ticking-off
So the sense is here that the author of the book writes some angry, nasty, negative, or at least critical things about this Swales person (whoever that is—someone who’s known not to be in good standing with the author, at any rate).
The word can also be used when referring to physical ‘rebukes’, to mean a sound thrashing:
He was attacked by some thugs on the way home from the pub the other night. Got a real coating too, by the sounds of it.
I have always thought that this sense comes from the use of coating to mean a layer of paint on a wall: the person who gets the real coating is figuratively covered in a thin layer of punches or criticisms. Thinking more closely about it, I realise it might just as well be an extension of the normal meaning of a coat, i.e., the person is ‘dressed’ in punches and criticism, wearing them like a coat.