Honorius quotes are cool sometimes, too.

Golem's Eye, p. 401: "'Look at that sunset!' it sighed, as if to itself. 'Like blood and melted cheese.'"
Golem's Eye, p. 405: "'Give me back my face!'"

This may not really be the funniest but is my personal favourite and always jumps to my mind.

"Seven planes of existence are good enough for anybody. Those who operate on more are just showing off."

That one jumps out to me too, although it isn't particularly funny.

Another jumper-outer:

The hamster nooded modestly. "I imagine that it didn't take long. There must be countless references to my exploits."
"In fact it took almost a year to find the slightest mention. I got the names of plenty of other demons of all sorts here and there among the library books. Nouda the Terrible kept coming up, as did an afrit named Tchue, and something called Faquarl was notable too in a dozen cultures. And then at last you appeared-- a fleeting mention in a footnote."

the five pebble line all readey mentioned

I liked in Ptolemy's Gate when Bartimaeus is in frog form and almost dead, and he sees Jane Farrar with John and says, "Is this your bird? Tell me she's not. Oh dear." Since I'm American I didn't know what he meant by "bird", but I had a guess. I looked it up in the dictionary and saw that I was right.

I like this one from PG:

"Our raid on the Ambassador Hotel was planned with military precision and utmost care. Just ten minutes' bickering in a phone box and we had the plan set straight." p. 252, Ch. 19