it actually depends on the course and the lecturer. i actually didnt buy two of my books in first year as they werent exactly needed, we only had to have them for tute questions because the actual text in the book was all crap compared to the lecture notes we were given. so it depends if the lecturer is dependent on the text or not. and one more thing second hand books will save you alot, i was stupid and brought brand new ones when second hand ones work the same and cost you less..
hope that helps