Here are some titles that in my opinion are essential for any working numismatic library:
A Guide Book of U.S. Coins, first published in 1946 with a cover date of 1947 (to prolong shelf life in book stores) can be collected across many editions for a low cost. It is interesting to compare prices. Did you know, for example, that many commemorative silver coins today sell for tiny fractions of the prices they commanded 60 years ago? It is true!
Early American Coins by Sylvester S. Crosby, 1875, is still a standard reference today. Amazing it would seem, but not at all surprising when you examine its contents. Buy or borrow a copy and spend an afternoon and evening absorbing its contents.
Early American Cents by Dr. William H. Sheldon, 1949, set forth many ideas and standards still in use today. The introductory paragraphs of this book are just as exciting to read today as they were many decades ago.
The Standard Catalogue of United States Coins, 1932 to 1958, provides a window on that seminal era.
The American Numismatical Manual by Dr. Montroville W. Dickeson, 1859, with revised editions in 1860 and 1865, is a nostalgia trip.
Just about any book by Walter Breen was viewed as Gospel by many if not most collectors, until his reputation was destroyed by personal indiscretions. He died in prison in 1993. Today, his books are still a rich source of information.
Everyone should build a basic library. This is easier in theory than in practice for many people.