The reference is to Kugler, ed. Eastlake, Handbook of the History of Painting, Part One, The Italian Schools, translated by 'A Lady', i.e. Lady Eastlake, First edition, London: John Murray, 1842; second and third editions revised by J.Burckhardt, London: John Murray 1847 and 1855.
Kugler, in his introduction to the English edition, claimed to be publishing a' short and easily-intelligible guide, pointing out to the unlearned the leading styles of Art, and serving as an introducion to the researches of our most recent writiers'.
Eastlake, in his introduction, called it an 'unpretending but useful little Hand-Book, the perusal of which would be a fit preparation for a visit to the collections of paintings on the continent and in our own country'. Eastlake's major reservation seems to have been that 'the indulgence with which the early Italian masters are regarded may sometimes, perhaps, convey too exalted an idea of their merit'. He picks out as an example the 'extravagance' of Kugler's praise of Duccio's Maesta.
Haskell, History and its Images, p. 331, calls Kugler's work 'long, learned and enormously influential'.