Short answer: List what you need, decide new or second hand for each, and set a fixed budget.
List
List every book you need for the year.
Decide
Mark which can be second hand and which must be new.
Track
Track spending and sell finished books to fund the next set.
Frequently asked questions
How much should I budget?
It depends on your course; start from the list.
Can I recover costs?
Yes by selling books you finish.