Upload & Sell: Off
Micky Bill focuses only on the value of the license agreement, not your original question which asked for pointers. You will probably be looking at a worldwide licensing agreement with potential publication in multiple subsequent editions. I doubt anyone on this list will have the expertise to give you good advice on this. The best pointer I can think of -- get a lawyer!
In reality textbook publishers usually plead extreme poverty even though they are usually very profitable due to the required purchases by students and the inflated cost of textbooks. Is it for digital/web/online use or actual books?
There are online price calculators where you plug in the amount of photos, where they will be used, the intended press run and the time period.
In fact there are many here who can give advice, maybe pick a little from each one. A run of the mill general lawyer will not be much help, a lawyer who specializes in IP, RM and the photography / publishing industry would be beneficial but then you'd probably spend $5000 to do a $1000 deal.