In the regular series, the key date is the 1885, followed by 1886 and 1912-S. Most 1885s are available in very low grades,
or as Proofs. Mid-grade business strikes are difficult to locate. Other coins sometimes considered semi-keys include 1888 and 1894.
All in all, Liberty Nickels are a readily obtainable series to complete, with some diligent searching for the keys and semi-keys.
A clandestine issue closed the series, with 5 pieces dated 1913. Although technically not a coin, the 1913 Liberty Nickel remains
one of the most valuable numismatic issues in the entire U.S. series.
For information on grading Liberty Nickels, please see the Grading Liberty Nickels page.
Although the BCCS had performed Rarity Surveys of the silver series in the 1990s, the Liberty Nickels were not originally included.
When the Society updated the studies with more grade ranges and added the the Census, Liberty Nickels were added.
Look at the Survey results for Liberty Nickels:
Liberty Nickel Census study
Liberty Nickel Rarity Ratings
See also the web site for the Liberty Nickel Collector Society.
One reference on this series is The Complete Guide to Shield and Liberty Nickels, by Gloria Peters and Cynthia Mohon,
with detailed information about the series, and date-by-date analysis.