The volumes in this series are the extant letter books of the publishing house. They contain copies of letters relating to the business of the publishing house, including letters to authors, editors, printers and other publishers. Only key letters were recorded, rather than every letter issued by the publishing house. As such, the volumes do not provide a comprehensive record of all letters issued by the publisher.

Two types of letter book are represented: copy letter books, which contain handwritten copies, often just summaries, of the key correspondence; and wet-press letter books, which contain full copies of letters reproduced through a duplicating process.

Folios from the letter books were occasionally removed, such that numbers may be missing from the original foliation sequence, or a stub only may be visible. Such letters are sometimes found with related correspondence in the Incoming Correspondence, or elsewhere in the archive.