I wanted to share this cool book with you all, and thanks to the "interwebs", I can do just that!
I picked up a copy at my local U's library, but it's archived on archive.org in all its glory.
George E. Brown's "Ferric & Heliographic Processes"
The most interesting thing about the book is that it contains actual cyanotypes. Look on page 17, and there are others as well. Interestingly, my copy has different images.
Anyways, this book is over a hundred years old now and I can attest to the fact that the prints look absolutely amazing, no deterioration whatsoever. Granted they've been in the dark, but it really says something about the durability of cyanotypes.
I have two cyanotypes made prior to 1900 on my wall. They still exhibit beautiful blue tones which they have done for the 30-40 years I have owned them. They seem to be absolutely archival. Think of blueprints. Did you ever see a faded one? same process.