In the United States, Gillette (the company founded by King Gillette) is the most familiar manufacturer of safety razors and blades. King Gillette is often credited with making popular the concept of disposable products. In the case of vintage razors, specifically the razor blades (as against the modern disposable razors and cartridges that still carry the Gillette name).
When collecting vintage razors, there are two publications that are, in my opinion, the authorities on the subject.
The first is Philip L. Krumholz’s exhaustive, informative, and entertaining “The Complete Gillette Collector’s Handbook”. Sadly, the book is no longer in print, but it does appear that Mr. Krumholz makes a digital edition available via eBay from time to time (his eBay handle is razorman2). The book covers the history of the company, a little bit of the Gillette family, and provides a lot of information from razors to other collectibles, with photographs for collectors. His book is in black-and-white.
The other is Robert K. Waits’ “Safety Razor Compendium” which covers much more than Gillette (such as Gem, Star, Schick, and others), as well as some non-U.S. manufacturers. Originally, it was only available on CD. I believe it may still be available online. The Compendium is definitely image-rich and includes many color photos of razors and razor advertising. Together with Mr. Krumholz’s book, one can pretty easily identify the majority of razors.
That said, I do not wish to detract from other experienced collectors nor, in any way, imply that my statements here carry more weight. However, these two gentlemen put in years of time and research and believed enough in their work to publish. In terms of collecting, there is value in having a set of criteria upon which one can rely. While razor identification, particularly with Gillette, can be something of a guessing game at times, I strongly prefer to defer to those who’ve done the legwork and publicly backed it with their reputation. Until another collector or historian is able to put together something comprehensive and stand behind it as a publication, I look to these two authors to settle questions of identification. There are many resources on the web and many other collectors with as much, or more, passion as I bring to this hobby. (In the end, it is only a hobby, and a hobby should be something one enjoys. Not something one argues about. Much.)