I totally agree - not only saving money, also environmental friendly.
However, the point Teresa mentioned is correct. They always need to do the exercises & small "jobs" included in the books. It is not easy to be able to reuse the books!
When I was young my parents asked me to use my sister's books and I wasn't happy when I first heard it. After explanation from my parents and understand that using old things doesn't mean 'love you less', that's OK. My younger son uses my elder son's books, clothes etc. Now my youngest daughter even says thank you to her brothers when she gets nice clothing from the 2 brothers! So, it depends how you explain the situation.