A recent John Updike essay led me to some memorable excerpts from Ralph Waldo Emerson's journals, including a brilliantly apropos meta-quote on the act of citation itself. Thanks to  (RWE, 1867):
Quotation --- yes, but how differently persons quote! I am as much informed of your genius by what you select, as by what you originate. I read the quotation with your eyes, & find a new & fervent sense. ... For good quoting, then, there must be originality in the quoter --- bent, bias, delight in the truth, & only valuing the author in the measure of his agreement with the truth which we see, & which he had the luck to see first. And originality, what is that? It is being; being somebody, being yourself, & reporting accurately what you see & are. If another's words describe your fact, use them as freely as you use the language & the alphabet, whose use does not impair your originality. Neither will another's sentiment or distinction impugn your sufficiency. Yet in proportion to your reality of life & perception, will be your difficulty of finding yourself expressed in others' words or deeds.
And re the act of composition itself, as given by Updike (New Yorker, 4 Aug 2003) and again via  (RWE, 1869):
Good Writing --- All writing should be selection in order to drop every dead word. Why do you not save out of your speech or thinking only the vital things --- the spirited mot which amused or warmed you when you spoke it --- because of its luck & newness. I have just been reading, in this careful book of a most intelligent & learned man, any number of flat conventional words & sentences. If a man would learn to read his own manuscript severely --- becoming really a third person, & search only for what interested him, he would blot to purpose --- & how every page would gain! Then all the words will be sprightly, & every sentence a surprise.
Finally, on reading materials worthy of a human mind, from Society and Solitude, from  (1870):
Books --- Be sure, then to read no mean books. Shun the spawn of the press on the gossip of the hour. Do not read what you shall learn, without asking, in the street and the train. Dr. Johnson said, "he always went into stately shops" and good travellers stop at the best hotels; for, though they cost more, they do not cost much more, and there is the good company and the best information. In like manner, the scholar knows that the famed books contain, first and last, the best thoughts and facts. Now and then, by rarest luck, in some foolish Grub Street is the gem we want. But in the best circles is the best information. If you should transfer the amount of your reading day by day from the newspapers to the standard authors --- But who dare speak such a thing.