There is an abundance of 27s in the Thursday Next series.
In the first book, Thursday is a Literary Detective for SpecOps 27.
(text reads Wordage is our business Grammar is our game.)
I know there were more that I came across while reading, before I started formally recording them, but I've just started The Well of Lost Plots and came across this line:
I had this idea for a bumper sticker: 'A unicorn isn't for page twenty-seven, it's for eternity.'
and here's another one, referring to the car Miss Havisham is driving:
It was the 27 liter Higham Special.