HI Paul,
Congratulations on the assignment. Will they have any reference to you about the photographer and contact info? How much will the book sell for? How strongly do they want your book. Is this expected to have new editions or is this on the come without certain market. Would they have exluisive rights?
Notwithstanding this clarifications, I'd say that at a minimal $500, at merely $1:00 per copy. Although if this is a well regarded book and they will print your picture beautifully with credits, so they have a sense of esthetics and perofessionalism up to $1600 seem fair.
Also look at how mjuch you would charge for say a 16x10 print in a series of say 7-15 and then you might get a range of your own worth that you have already find comfortable.
I hope this might be useful and good luck. What's the book about? I'll look out for it!
That would make a nice short article: the book cover market!
Asher