There are been a fair few colouring books for grown up's on this site but The Executive Colouring Book, deserves a mention in its own right. As the original book is now out of print, scans have been uploaded so you can print out any of the pages you desire to colour. Worth a mention is also its footers at the bottom of each picture such as, "This is my train. It takes me to the office everyday." You meet lots of interesting people on the train. Colour them all in grey. Found this on a Sunday night, so it all feels very familiar (unfortunatly).
Other coloring books for adults featured on Trend Hunter include: Gangster Colouring Books and the C__ Colouring Book.