The reviewer (Benjamin Markovits) opens with the observation that many poets become famous in pairs and that their friendships seem to matter more than the friendships of other writers. I'm not sure if the numbers ("many") really bear out this claim, and Markovits doesn't mention, for instance, Wordsworth and Coleridge. In any case, I like the reason he gives: "the pressure on poets to re-invent their tradition -- it is easier to make up a game if you have someone else to play it with." Ricks' book, True Friendship, offers an analyses of these games, with close textual readings. Ricks suggests (according to Markovits) that unconscious borrowings by poets are more interesting that conscious ones, "suggesting as they do the way a poet engages with other poets on a level deeper than his or her critical opinions."
The relationship between Goethe and Schiller was more "programmatic" than the relationships Ricks describes. We are talking about Weimar classicism, after all. A major difference with 18th-century writers, even supposedly "modern" ones like Goethe, however, is the extent of conscious (rather than the "deep levels" noted by Ricks) borrowing of literary predecessors. Earlier poets were quite proud of being part of a continuous literary lineage, and showed their "legitimacy" as poets by drawing on the heritage. Shakespeare didn't feel less of a writer of sonnets because Petrarch also wrote them. "Originality" -- suggesting lack of literary origins -- first came about in the 18th century, but it was not something that earlier generations of poets would ever have been proud to claim about themselves.
I think that what distinguishes Goethe from earlier writers is that he went out of his way to hide the traces of the influence of earlier writers. There is a well-known scene in The Sorrows of Young Werther, in which Lotte and Werther witness a violent thunderstorm. She exclaims, "Klopstock!" This exclamation refers to a poem by Klopstock admired by everyone with any feeling in the 1770s. As Werther writes, recounting the moment, "I remembered at once that magnificent ode of his which was in her thoughts, and felt overcome by the flood of emotion which the mention of his name called forth. It was more than I could bear. I bent over her hand, kissed it in a stream of ecstatic tears, and again looked into her eyes. Divine Klopstock! If only you could have seen your apotheosis in those eyes!" (Victor Lange and Judith Ryan translation).
Much has been written about the influence of Klopstock on the young Goethe, for instance, by Meredith Lee. Goethe, as he recounts in his own autobiography, read Klopstock avidly as a child. But the passage in Werther is followed by another comment that indicates that Goethe is ambivalent about this influence: "And your name [i.e., Klopstock], so often profaned, may I never hear it uttered again!"
This is "anxiety of influence" avant la lettre. Since the reign of "originality," all that is left is unconscious assimilation of both poetic and non-poetic factors.