A book in which letters are filed, or in which copies of letters are made, for preservation.(noun)
The Century Dictionary (Public Domain)
Use "letter-book" in a sentence
"Simpson sold a very important manuscript, the original letter-book of"
"It proved to be fragments of the missing letter-book of Governor Bradford."
"One of my aides (Captain Dayton) acted as assistant adjutant general, with an order-book, letter-book, and writing-paper, that filled a small chest not much larger than an ordinary candle-boa."