Most of the books for The Simpsons are either about Bart, The entire family or just comics.

Anyway, I usually work on an up front basis for most things, but it depends on how my own personal finances are at the time before I can say anything for sure. If I'm low, then it'll have to be an up front payment, if I'm okay for it, then paying me back would suffice.
However, I do only accept online payments via Paypal.