Milles' infamous book was published in 1782, but the copy, shown above, was presented by him to James 'two pistols' Thistlethwaite, and is annotated 'The Dean of Exeter to Mr Thistlethwaite, 12th Dec 1781'.  I suppose it is fair to say that the book was gifted to Thistlethwaite in return for the first hand information he supplied regarding Chatterton.

The Dictionary of National Biography, has a paragraph detailing Milles' misguided attempt to prove that Chatterton's poems were written by Rowley;  I quote the whole paragraph below, all as written:

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•   "Unfortunately for his reputation Milles rushed into the Chatterton dispute with an extravagant edition of ‘Poems supposed to have been written at Bristol in the Fifteenth Century by Thomas Rowley, Priest. With a Commentary,’ 1782, copies of which, with numerous manuscript notes by Haslewood, Dr. Sherwen, and Horace Walpole, are in the British Museum. In this work he maintained the antiquity of the poems, and committed himself to the assertion, when writing on the poem of the death of ‘Syr Charles Bawdin,’ that ‘a greater variety of internal proofs may be produced for its authenticity than for that of any other piece in the whole collection.’ His ingenuous comments provoked replies from Edmund Malone, Thomas Tyrwhitt, and Thomas Warton, and a very severe ‘Archæological Epistle to Dean Milles,’ 1782, which, though long attributed to the poet Mason, was written by John Baynes [q. v.] On the dean's part in this controversy S. T. Coleridge wrote that he ‘foully calumniated Chatterton, an owl mangling a poor dead nightingale,’ and that ‘though only a dean, he was in dulness and malignity most episcopally eminent’ (Joseph Cottle, Early Recollections, i. 36)."