Robert Waits — A Safety Razor Compendium
The best available resource for vintage razors. I recommend the digital PDF version as you can search through it. It is available here. The book is not without errors and my list of errata can be found here.
Robert Waits — Before Gillette
Another great book by Robert Waits about the history of the Safety Razor from 1762 to 1901. A digital copy is available here.
Howard Hazelcorn — Revised Guide to Kampfe Bros. Wedge Blade Star Safey Razors
The Standard work on Kampfe Razors. There are three editions and all of them are as rare as hen's teeth as they were privately published in small numbers. Good luck finding a copy.
Renzo Jardella — Star Safety Razors and Accessories 1875-1919
A lavish book with great illustrations. The author occasionally sells copies on eBay.
Useful links when researching vintage patents
European Patent Office — https://worldwide.espacenet.com
My goto resource for vintage patents. Great search facilities and worldwide coverage.
Google Patents — https://patents.google.com
Google's search engine for patents. Quite good, but I prefer the EPO's user interface. Quite often Google links to the EPO website for details.
U.S. Patent Office — https://www.uspto.gov
This site also has trademark information, but the user interface is often confusing and it only covers U.S. documents.
The Lens Patent Search — https://www.lens.org/lens/search/patent/structured
A good resource when searching by inventor names.
PatBase — http://www.patbase.com
A commercial patent database, but they have a free “patent classification explorer”.
Patent History Materials Index - https://ipmall.law.unh.edu/content/patent-history-materials-index
From the University of New Hampshire. Lots of original patent resources from the 19th century.
They also have the complete text of A History of the Early Patent Offices: The Patent Office Pony by Kenneth W. Dobyns
Useful links when researching historical information
FamilySearch — https://familysearch.org
A very good website run by the Mormon Church. First port of call for births, marriages, immigration, census information and deaths.
Some of the information is not accessible from the internet, but only from affiliated libraries. My local UK library is one of them. You can find documents using the computers at the library and email them to yourself.
Find A Grave — https://www.findagrave.com/
Good for deaths, but only good for deaths.
FreeCen — https://www.freecen.org.uk/
UK census information up to 1891.
Ancestry — https://ancestry.com
Another website for finding historical documents. However, it's subscription only and quite expensive.
One-Step Webpages — https://stevemorse.org/
A great free resource for U.S. immigration records, census records including information about the ships carrying immigrants to the U.S.
New York Times archive
I can no longer recommend the New York Times archive as a source.
It's a paid subscription that is almost impossible to cancel.
The venerable New York Times has turned into a scam.
Newspapers.com — https://www.newspapers.com/
You can search thousands of newspapers for names of individuals or companies. It's a paid subscription, but it can be cancelled anytime without problems.