"Booksellers were often publishers, and publishers were often printers" (435).
Publishers tended to specialize in the kinds of materials they published, but printers tended not to, although there were some notable exceptions. Thomas East was known for printing music, and William Jaggard for heraldry, although neither of these printers confined themselves strictly to their respective materials (435 - 436).
"Many authors did not prepare their manuscripts for the press, and, on several occasions, their texts were printed without their knowledge" (438).
The German medical student and sometime proofreader Hieronymus Hornshuch complained that "it is particularly bothersome if someone submits his writings to their press not neatly written" (439). Henry Chettle's explanation that Greene's handwriting in Groatsworth of Wit was difficult to read, and thus Chettle transcribed Greene's manuscript himself (439). Thomas Heywood asks the reader to excuse the compositor at the end of The Exemplary lives and Memorable Acts of Nine the Most Worthy Women for setting type from a manuscript "coppy in a difficult and unacquainted hand" (439). These three testimonials offer evidence that not every manuscript was perfected before being submitted to the printer.
Moxon's description of proof reading was a three stage process, and while he is to some extent describing an ideal printshop, Moxon was himself a printer, and his descriptions of this practice are ikely at least partially based in his experience (443).
Heywood complains in Exemplary Lives of having to include an errata list because the "corrector... could not bee always ready in regard of some necessary employment" (443).
Topsell admits in the introductory epistle to his History of Serpents that numerous errors in his previous book, Four-footed Beasts were present because he and the publisher "were not so thoroughly estated as to maintaine a sufficient scholler to attend only upon the presse" (443).
"That it was usual for authors to correct proofs is indicated by the apologies for their failure to do so" (443).
Proof reading is unquantifiable, but the evidence of printed texts suggests that it was common practice, and according to Moxon the print shop corrector was assisted by a reader who read aloud from the manuscript (444).
Proofreading was undertaken to correct mechanical errors, such as wide white spaces, unlinked white spaces, ink splotches, and misaligned type (444).
Maguire, Laurie E. "The Craft of Printing (1600)." A Companion to Shakespeare. David Scott Kastan Ed. Malden: Blackwell. 1999. p 434 - 449. Print.