Page 242. " 'Hello Jack Frost,' he said "

Jack Frost is a figure from English folklore, the personification of ice and frost. 

Page 243. " Jack Dandy's smile grew wider "

Dandies, 1831
Public DomainDandies, 1831 - Credit: Journal des Dames et des Modes
Jack-a-dandy, a variation upon "dandy", is a contemptuous name for a male who is overly concerned with his appearance and dress.  It was commonplace from the mid-sixteenth to the mid-nineteenth centuries. 

It is also the name of a character in a comic book written by Alan Moore, one of Neil Gaiman's literary influences.

Page 244. " Jack Ketch, with a twitch of his moustache "

Jack Ketch was a famous seventeenth century hangman, whose name became a nickname for all hangmen and, by extension, a euphemism for death, particularly by hanging.

Page 244. " 'The boy?' asked the Jack called Nimble "

Jack Be Nimble
Public DomainJack Be Nimble - Credit: William Wallace Denslow
 Jack Be Nimble is an old English nursery rhyme:

 Jack be nimble,

 Jack be quick,

 Jack jump over

 The candlestick.