Dick Steven's book is the only thing curently in print. There are
many discussions in the literature of the teens & twenties, at
least in American publications (esp. Photo-Miniature) and at least
a few in Germany (I"ll maul this: Photographisce Mittilungen).
Most methods of altering cyanotype colours are fairly unstable.
In over 20 years of fooling with them, the only simple & stable
method has been *selenium toning* with Kodak Selenium Toner.
Make a fairly dilute bath, submerge the cyanotype, agitate gently,
watch the colour shift. Be careful because if you leave it in too
long, it will bleach the image.
Enjoy!
Russ Young
Santa Fe, New Mexico
USA