A glass man is a creature in The Inkworld.
Many of the civilised city-dwelling glass men work for scribes. They sprinkle sand over the ink that their master's write, and prevent the words from smudging. They do other simple jobs like sharpening pens and making sure their masters's pieces of work are perfect. Glass men also stir up ink and to make sure that the ink has been mixed for long enough, they have to breathe 165 times while doing it. Only then, will the ink be perfect.
In Inkdeath, Farid is forced to work for Orpheus and while doing so, he meets Orpheus's glass men: Ironstone and Jasper. Ironstone is Orpheus's favourite and bullies his younger brother, Jasper. Farid feels sorry for him and, every now and then, secretly takes Jasper out to Seamstresses' Alley, where Jasper watches the glasswomen weave and sew dresses for their mistresses and Farid learns that glassmen have romantic feelings too. He learns this when Jasper falls deeply in love with a glasswoman whom he spies on when she's working: looking through her mistresses's workshop, just to see her weave.
Farid also learns that glassmen bleed. When Jasper was sharpening a pencil once ( a very unpleasant and dangerous job for a glassman as the blade can cut them), the sharp blade dug into him and he bled. But, instead of red blood, he bled transparent blood. This is because all glasspeople's blood are the colour of glass: just like their bodies.
Glassmen also eat sand.